[3.x] Add horizon specular occlusion

[clayjohn]

Implements horizon specular occlusion. This helps reduce artifacts where you see the sky on the edges of e.g. tiles bricks etc.

before: GLES3

after: GLES3

before: GLES2

after: GLES2

Note: for some reason GLES2 suffers much less from this particular artifact. With horizon specular occlusion it looks almost perfect. At some point it may be worth exploring what is going on to see if we can improve GLES3 similarly.

[Calinou]

Looks great! Does this take SSAO into account as well, or does it even rely on AO maps in the first place? Either way, I think this supersedes #50603.

Note: for some reason GLES2 suffers much less from this particular artifact. With horizon specular occlusion it looks almost perfect. At some point it may be worth exploring what is going on to see if we can improve GLES3 similarly.

My guess is that the radiance/irradiance map is less detailed in GLES2 (and possibly darker), due to using a more approximate way to generate it. This would make the artifacts less noticeable overall.

[clayjohn]

Looks great! Does this take SSAO into account as well, or does it even rely on AO maps in the first place? Either way, I think this supersedes #50603.

Ah! I forgot your PR included horizon specular occlusion. This is just horizon specular occlusion and does not touch regular AO at all.

The big difference here is that the horizon specular occlusion is only factored in for the IBL and not for other sources of specular lighting.

[akien-mga]

Thanks!