Remove shadow direction quantization, increase shadow frames instead

[lhofhansl]

Ever wondered why shadows are laggy even when you set shadow_update_frames to 1?
This most visible at dusk and dawn when shadows change quickly due to the angle of the sun.

This has bothered me for quite a while, so I finally tracked it down.
This was due to the direction quantization we do, which was added to reduce the jittering in the shadows (if it is updated less frequently you do not notice the jittering so much).

I had an earlier version where I quantize less during dusk and dawn, but in the end I think we should just remove it. If we really wanted to I can bring quantization back around midday and midnight whrere shadows move the least at the jittering would be most obvious)

The old logic shows a different shadow every PI/2880 change, at a day length of 20 minutes that comes to about 0.2s (20*60/2/pi/2880 = 0.2083...), which matches my observation.

So we go through all the work calculating shadows, etc, just to not show them more than 5 times/s.

Instead we roughly get the same effect if shadow_update_frames is set to 16 (assuming 60 FPS).
For nice machines and larger shadow texture, folks now reduce this to get smooth shadow movement.

• Goal of the PR
  Allow for smoothly changing shadows.

• How does the PR work?
  Removing sun/moon direction quantization, replace it by increasing shadow update frames.
  And that allows getting smooth shadow movements on better machine by reducing shadow update frames.

• Does it resolve any reported issue?
  Nope

• Does this relate to a goal in the roadmap?
  Nope

• If not a bug fix, why is this PR needed? What usecases does it solve?
  Shadows were laggy.

To do

This PR is Ready for Review.

How to test

Enable shadows. Pick a good view distance, set time to dusk/dawn and observe shadows moving in a choppy way.
Assuming 60FS this should be roughly the same before and after.

Now reduce shadow_update_frames to 1 and enjoy smoothly moving shadows.

[tigercoding56]

i think we should make it a option, especially now that sfan5 removed some options with the latest commit, we need something to fill the settings screen
i propose 3 options:
less directional quantisiation in certain situations (shadows that don't change quickly) (partial)
full directional quantisation (full)
and no directional quantisation (disabled)

[sfan5]

makes sense.

[lhofhansl]

quantize less during dusk and dawn, but in the end I think we should just remove it

Changed my mind. Now shadows are quantized around noon and midnight, and in a way that does not waste resources.
Removed the "One Approval" since the code changed.

@sfan5 Would you have another look?

[lhofhansl]

float comparison is OK here, since quantizeDirection uses ceil for integer math.

[SmallJoker]

Suggested change

	v3f last_quant_directin{0,0,0};
	v3f last_quant_direction{0,0,0};

EDIT: or

Suggested change

	v3f last_quant_directin{0,0,0};
	v3f last_quant_direction;

(use default constructor)

[SmallJoker]

What if set_sky is used, where the sun is tilted? The shadows would still change noticeably during these time limits.

[sfan5]

I don't see why we should quantize the direction at all?
Instead increasing shadow_update_frames makes it equivalent, more performant and user-configuable.

[lhofhansl]

@sfan5 ok... I'll revert the last two commits and then merge (the changes you had already reviewed)

[lhofhansl]

OK. Reverted the last two commits and re-added @sfan5's original approval of the first commit.