Fixed shadow volume regression for mobile

[IanLilleyT]

Fixes #8953

#8854 clamped vertices to the near plane in order to create a watertight shadow volume. While this happened to work well for orthographic mode, and worked sometimes on desktop (curiously, only when ANGLE was enabled), it broke down for mobile, and was flawed to begin with. This is because the way we do shadow volumes doesn't actually require the front of the volume to be closed, just the back.

Ultimately we want a solution that emulates GL_DEPTH_CLAMP, which is not available in WebGL 1 or 2. GL_DEPTH_CLAMP clamps geometry that is outside the near and far planes, capping the shadow volume. More information here.

We emulate GL_DEPTH_CLAMP by ensuring no geometry gets clipped by setting the clip space z value to 0.0 and then sending the unaltered screen space z value (using emulated noperspective interpolation) to the frag shader where it is clamped to [0,1] and then written with gl_FragDepth. This is the solution when GL_EXT_frag_depth is available and works for both perspective and orthographic. It is based off of https://stackoverflow.com/questions/5960757/how-to-emulate-gl-depth-clamp-nv.

When GL_EXT_frag_depth is not available, which is the case on some mobile devices, we must attempt to fix this only in the vertex shader. Our approach is to clamp the z value to the far plane, which closes the shadow volume but also distorts the geometry, so there can still be artifacts on frustum seams but it won't be as severe as the code in #8854 which also clamps the near plane unnecessarily.

Before:

After:

Sandcastle

Here are some examples of frustum seam artifacts that could not be fixed in this PR when GL_EXT_frag_depth is not available. These artifacts have existed since shadow volumes were first implemented:

Sandcastle for testing more options

@lilleyse and I worked on this fix together so someone else should review. @kring?

CC @OmarShehata

[cesium-concierge]

Thanks for the pull request @IanLilleyT!

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[Brightboy666]

Banga

[lilleyse]

This PR fixes some other artifacts when log depth is disabled:

Sandcastle

[dennisadams]

This PR fixes some other artifacts when log depth is disabled:

This is certainly an improvement but I still have some problems on my Android:

Before:

After:

[lilleyse]

Ah I should've mentioned I had only tested the ellipse sandcastle on desktop linux where it is now fixed. I see those same artifacts on Android where frag depth isn't supported (in the WebGL 1 context at least).