EEVEE: Occlusion: Use ScreenSpaceRay for iteration

The sampling is now optimum with every samples being at least one pixel
appart. Also use a squared repartition to improve the sampling near the
center.

This also removes the thickness heuristic since it seems to remove
a lot of details and bias the AO too much.

EEVEE: RenderPass: Improve AO pass if screen space radius is small

This just bypass the occlusion computation if there is no occlusion
data. This avoids weird looking occlusion due to the screen space
geometric normal reconstruction.

EEVEE: Ambient Occlusion: Add sample parameter support for the AO node

The actual sample count is rounded up to a multiple of 4 because we
sample 4 horizons directions.

Changing this setting forces the shader to recompile (because using a
GPU_constant).

Fix T81741 EEVEE: Ambient Occlusion does not converge properly

This was due to the AO random sampling using the same "seed" as
the AA jitter. Decorelating the noise fixes the issue.

EEVEE: Use Fullscreen maxZBuffer instead of halfres

This removes the need for per mipmap scalling factor and trilinear interpolation
issues. We pad the texture so that all mipmaps have pixels in the next mip.

This simplifies the downsampling shader too.

This also change the SSR radiance buffer as well in the same fashion.

EEVEE: SSRayTrace: Cleanup/Refactor

This is a major rewrite that improves the screen space raytracing
a little bit.

This also decouple ray preparation from raytracing to be reuse in other
part of the code.

This changes a few things:
- Reflections have lower grazing angle failure
- Reflections have less self intersection issues
- Contact shadows are now fully opaque (faster)

Unrelated but some self intersection / incorrect bad rays are caused by
the ray reconstruction technique used by the SSR. This is not fixed by
this commit but I added a TODO.

EEVEE: Sampling: Split hemisphere sampling just like GGX

This is useful for debugging raycasting.

Merge branch 'master' into tmp-vulkan

Merge branch 'master' into tmp-vulkan

# Conflicts:
# source/blender/blenkernel/BKE_global.h
# source/blender/gpu/intern/gpu_context.cc
# source/blender/windowmanager/intern/wm_window.c
# source/creator/creator_args.c

Fix T86042 EEVEE: incorrect irradiance bake

The environment (world) irradiance wasn't correctly skipped.