Issue #20623: Check the input to WebGLRenderingContext's clear().
Validate the input to this function as per specifications.
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- [X] `./mach build -d` does not report any errors
- [X] `./mach build-geckolib` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#20623
- [X] There are tests for these changes
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