Fix potential cuda initialization error

[HolyWu]

When using cuda hwaccel, it's likely to hit the following error on high core count systems due to exhausting the maximum frame pool size. Using too much threads for nvdec doesn't give significant performance gain, especially for h264. Hence set the default thread count to 1 for h264, and a maximum of 2 for all the other codecs.

[h264 @ 0000016418585440] decoder->cvdl->cuvidCreateDecoder(&decoder->decoder, params) failed -> CUDA_ERROR_INVALID_VALUE: invalid argument
[h264 @ 0000016418585440] Using more than 32 (46) decode surfaces might cause nvdec to fail.
[h264 @ 0000016418585440] Try lowering the amount of threads. Using 16 right now.
[h264 @ 0000016418585440] Failed setup for format cuda: hwaccel initialisation returned error.