Fix support for SV_Coverage on GLSL path (#1239)

There were two overlapping issues here:

1. We always mapped `SV_Coverage` to `gl_SampleMask`, even though `gl_SampleMaskIn` is the correct built-in variable to use for an input varying.

2. We treated `gl_SampleMask` like it was a scalar shader input, when it and `gl_SampleMaskIn` are actually arrays of indeterminate size (as a byproduct of trying to future-proof for implementations that might support hundreds or thousands of samples per pixel...)

The fix here is simple: map to either `gl_SampleMask[0]` or `gl_SampleMaskIn[0]` as appropriate. I suppose that this approach doesn't handle the possibility of eventually supporting >32 samples per pixel by having something like `uint2 coverage : SV_Coverage`, but I think we can cross that bridge when we come to it.

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