[3.x] Add support for 16-bit directional and point light shadow atlases

[Calinou]

Follow-up to #54165.

This results in slightly faster rendering with no discernible visual difference in most scenes. Both GLES3 and GLES2 support this option, which is also enabled by default.

Please test on various platforms, especially Android, iOS and HTML5 🙂

Testing project: test_shadow_16_bits_3.x.zip

Preview

Tested on a GeForce GTX 1080 in 2560×1440.

The performance difference is very slight here, but in a more complex scene with more lights, it would be more significant.

GLES3

24-bit	16-bit	Difference (dssim)

GLES2

24-bit	16-bit	Difference (dssim)

[lawnjelly]

I wonder whether 16 bit / 24 bit may be quite scene dependent on whether it makes a visual difference, depending on the near and far planes of the shadow map "camera" and the relative size of the polys. I guess it might affect things like shadow acne.

Worth mentioning another option is just to have a global 16 bit setting, rather than overridable per viewport. That way you could probably do the PR with one line of code or so, just change storage->config.depth_buffer_internalformat on startup according to a project setting. I guess having it per viewport is more adaptable though.

I have to say I'm not totally convinced on having it 16 bit by default, especially at this late stage in 3.x. Quite apart from the risk that it may make some types of scenes look worse, we are also assuming here that all GPUs will always run faster with 16 bit than 24 bit, we'd need a GPU guy to confirm this (there are all kinds of counter intuitive things that can go on). Maybe some support 32 bit GL_OES_depth32 I don't know whether we are able to take advantage of this currently, just throwing that out there.

Also it would be helpful to know what kind of performance differences to expect, have you got any test results for different platforms? Especially mobile that supports the 24 bit extension. Obviously if there is a large difference it is more convincing to change the default, but if it is marginal, maybe it would be less worth it.