gl.SAMPLE_ALPHA_TO_COVERAGE support

[takahirox]

#12438

This PR adds gl.SAMPLE_ALPHA_TO_COVERAGE support. User can control it with material.alphaToCoverage.

I'm not familiar with sample coverage, do you folks think we also need gl.SAMPLE_COVERAGE and gl.sampleCoverage()?

/cc @mattdesl

[Mugen87]

I think this PR might get more traction if an example would demonstrate the effect of enabling SAMPLE_ALPHA_TO_COVERAGE. A tree or some other sort of vegetation might be a good starting point. Right now, it's a bit hard to understand the benefit of providing this new material property.

[Mugen87]

Closing for now. Let's reopen the PR when an example is ready.