Auto merge of #20754 - anholt:webgl-fixes, r=emilio

Webgl fixes

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Add gl.getParameter() support for webgl unpack settings, and remove some unreachable and incorrect error-generation code

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<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #20372
- [x] These changes fix #20553
- [x] These changes fix #20554

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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tests/wpt/mozilla/meta/webgl/conformance-1.0.3/conformance/state/gl-get-calls.html.ini

@@ -57,12 +57,6 @@
   [WebGL test #47: context.getParameter(context.SCISSOR_TEST) should be false (of type boolean). Was null (of type object).]
     expected: FAIL
 
-  [WebGL test #85: context.getParameter(context.UNPACK_FLIP_Y_WEBGL) should be false (of type boolean). Was null (of type object).]
-    expected: FAIL
-
-  [WebGL test #86: context.getParameter(context.UNPACK_PREMULTIPLY_ALPHA_WEBGL) should be false (of type boolean). Was null (of type object).]
-    expected: FAIL
-
   [WebGL test #88: context.getParameter(context.VIEWPORT) is not an instance of Int32Array]
     expected: FAIL