This PR sets up the ability for shaders to use `gl_ClipDistance`, which will be needed in the future. Desktop GL supports this natively. OpenGL ES requires the EXT_clip_cull_distance extension.
Unfortunately glslang does not support this extension, so we have to employ a workaround for mobile when going through glslang. We instead write to `filament_gl_ClipDistance`, and then modify the SPIR-V to decorate this as `gl_ClipDistance`. See the comment in SpirvFixup.h.
Note this PR does not actually use `gl_ClipDistance` yet, so there should be no change to shaders.
