header: implement frame_size_with_refs()

[jamrial]

The spec has a code path in the uncompressed header named frame_size_with_refs(), defined in section 5.9.7, which an inter frame may use to infer its dimensions from one of the seven frames it uses as reference instead of inferring them from the sequence header or ultimately hardcoding them. This path is triggered on non error resilient encodings by signaling frame_size_override_flag == 1, a bit that means that either frame dimensions are coded in the frame, or that an attempt to infer both frame and render dimensions from a reference frame should be done.

For encodings using a SAR other than 1:1, rav1e is currently hardcoding the render sizes on every frame, which is an overhead of about 4 bytes. With this change, for inter frames the encoder will look at each of the seven reference frames and if it finds one where both frame and render sizes are the same as this frame, instead of writing the sizes it will write the bits needed to tell the decoder to infer them from it.

Here's a comparison of the uncompressed header written for inter frames before and after this PR when encoding a 720x480 sample with SAR 2:1.

Before

[trace_headers @ 00000203eb3bfda0] Frame Header
[trace_headers @ 00000203eb3bfda0] 16          show_existing_frame                                         0 = 0
[trace_headers @ 00000203eb3bfda0] 17          frame_type                                                 01 = 1
[trace_headers @ 00000203eb3bfda0] 19          show_frame                                                  1 = 1
[trace_headers @ 00000203eb3bfda0] 20          error_resilient_mode                                        0 = 0
[trace_headers @ 00000203eb3bfda0] 21          disable_cdf_update                                          0 = 0
[trace_headers @ 00000203eb3bfda0] 22          frame_size_override_flag                                    0 = 0
[trace_headers @ 00000203eb3bfda0] 23          order_hint                                             000011 = 3
[trace_headers @ 00000203eb3bfda0] 29          primary_ref_frame                                         010 = 2
[trace_headers @ 00000203eb3bfda0] 32          refresh_frame_flags                                  00100000 = 32
[trace_headers @ 00000203eb3bfda0] 40          frame_refs_short_signaling                                  0 = 0
[trace_headers @ 00000203eb3bfda0] 41          ref_frame_idx[0]                                          100 = 4
[trace_headers @ 00000203eb3bfda0] 44          ref_frame_idx[1]                                          100 = 4
[trace_headers @ 00000203eb3bfda0] 47          ref_frame_idx[2]                                          101 = 5
[trace_headers @ 00000203eb3bfda0] 50          ref_frame_idx[3]                                          100 = 4
[trace_headers @ 00000203eb3bfda0] 53          ref_frame_idx[4]                                          100 = 4
[trace_headers @ 00000203eb3bfda0] 56          ref_frame_idx[5]                                          100 = 4
[trace_headers @ 00000203eb3bfda0] 59          ref_frame_idx[6]                                          001 = 1
[trace_headers @ 00000203eb3bfda0] 62          render_and_frame_size_different                             1 = 1
[trace_headers @ 00000203eb3bfda0] 63          render_width_minus_1                         0000010110011111 = 1439
[trace_headers @ 00000203eb3bfda0] 79          render_height_minus_1                        0000000111011111 = 479

After

[trace_headers @ 000001e8e5d5fda0] Frame Header
[trace_headers @ 000001e8e5d5fda0] 16          show_existing_frame                                         0 = 0
[trace_headers @ 000001e8e5d5fda0] 17          frame_type                                                 01 = 1
[trace_headers @ 000001e8e5d5fda0] 19          show_frame                                                  1 = 1
[trace_headers @ 000001e8e5d5fda0] 20          error_resilient_mode                                        0 = 0
[trace_headers @ 000001e8e5d5fda0] 21          disable_cdf_update                                          0 = 0
[trace_headers @ 000001e8e5d5fda0] 22          frame_size_override_flag                                    1 = 1
[trace_headers @ 000001e8e5d5fda0] 23          order_hint                                             000011 = 3
[trace_headers @ 000001e8e5d5fda0] 29          primary_ref_frame                                         010 = 2
[trace_headers @ 000001e8e5d5fda0] 32          refresh_frame_flags                                  00100000 = 32
[trace_headers @ 000001e8e5d5fda0] 40          frame_refs_short_signaling                                  0 = 0
[trace_headers @ 000001e8e5d5fda0] 41          ref_frame_idx[0]                                          100 = 4
[trace_headers @ 000001e8e5d5fda0] 44          ref_frame_idx[1]                                          100 = 4
[trace_headers @ 000001e8e5d5fda0] 47          ref_frame_idx[2]                                          101 = 5
[trace_headers @ 000001e8e5d5fda0] 50          ref_frame_idx[3]                                          100 = 4
[trace_headers @ 000001e8e5d5fda0] 53          ref_frame_idx[4]                                          100 = 4
[trace_headers @ 000001e8e5d5fda0] 56          ref_frame_idx[5]                                          100 = 4
[trace_headers @ 000001e8e5d5fda0] 59          ref_frame_idx[6]                                          001 = 1
[trace_headers @ 000001e8e5d5fda0] 62          found_ref[0]                                                1 = 1

Currently, rav1e has fixed frame and render sizes for the entire video sequence, so it will always signal to use the very first reference frame, but with this change the frame header writing code is ready to handle both frame and render sizes changing in a per-frame basis, if it ever gets implemented.

[coveralls]

Coverage increased (+0.001%) to 83.093% when pulling f5f8b0a on jamrial:frame_size_with_refs into 47039d9 on xiph:master.

[lu-zero]

It looks fine to me, thank you.

[jamrial]

Can this be merged? Among other things, it's required to ensure #2490 works as intended.