Test textureSample WGSL validation

[greggman]

Looking for feedback / guidance since this is my first WGSL test - I'm not sure every case that should be tested.

textureSample takes a bunch of formats. I tested that the wrong type can't be passed to each parameter except for the texture (not sure how you'd pass an incorrect texture type since the function is determined from the texture type. I suppose I could test passing an external texture ? Similarly I didn't test not passing a sampler since there is only one type of sampler.

I wasn't exactly sure how to tell if types are compatible so I hacked in isCompatible. I wanted the test to at least have some success cases because if it only expects failure than any typo will cause it appear to be correctly failing all tests.

I also wasn't sure what to check for const-expression. The spec says the offset parameter must be a const-experssion so I made 3 variables c (a const) u an uniform, l a let expression based off something non-uniform. It works but I'm not sure if there is some more official way or if there are more types I should be testing. I was surprised a let expressions like let a = 1 is not considered const even though AFAICT it is actually const. I'm guessing that's in the spec but it seems unintuitive.

[ben-clayton]

Made some suggestions for how you can use the Type / Value utilities to remove the string comparisons.

[ben-clayton]

I was surprised a let expressions like let a = 1 is not considered const even though AFAICT it is actually const. I'm guessing that's in the spec but it seems unintuitive.

You want const a = 1 for that to be a const expression. A let and var usage is a runtime expression.

[ben-clayton]

Great job. One nit, but feel free to ignore.