Auto merge of #12794 - anholt:webgl-depthrange-validate, r=emilio

webgl: Validate that depthRange near <= far.

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Add a check for one of the subcases of webgl-specific.html.

I added a longer comment citing the spec than is common in the file -- we've found these kinds of citations really useful in Mesa, but if we want to keep it to the spec link, I could change it.

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- [ ] These changes fix #__ (github issue number if applicable).

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Fixes a subtest of webgl-specific.html.

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