MacOS IOSurface bug in glCopyTexImage2D causes WebGL test failrues

[asajeffrey]

According to https://searchfox.org/mozilla-central/rev/6cd54550a27e2f6ca0755a25328f769e41e524f4/gfx/gl/SharedSurfaceIO.cpp#39-44:

  /* Bug 896693 - OpenGL framebuffers that are backed by IOSurface on OSX expose
   * a bug in glCopyTexImage2D --- internalformats GL_ALPHA, GL_LUMINANCE,
   * GL_LUMINANCE_ALPHA return the wrong results. To work around, copy
   * framebuffer to a temporary texture using GL_RGBA (which works), attach as
   * read framebuffer and glCopyTexImage2D instead.
   */

referencing https://bugzilla.mozilla.org/show_bug.cgi?id=896693

This produces WebGL test failures using alpha and luminance with gl.copyTexImage2D, for example

  ▶ Unexpected subtest result in /_webgl/conformance/textures/misc/copy-tex-image-2d-formats.html:
  │ FAIL [expected PASS] WebGL test #2: should be 0,0,0,127\nat (0, 0) expected: 0,0,0,127 was 0,0,0,0
  │   → assert_true: should be 0,0,0,127
  │   → at (0, 0) expected: 0,0,0,127 was 0,0,0,0 expected true got false
  │ FAIL [expected PASS] WebGL test #5: should be 64,64,64,255\nat (0, 0) expected: 64,64,64,255 was 0,0,0,255
  │   → assert_true: should be 64,64,64,255
  │   → at (0, 0) expected: 64,64,64,255 was 0,0,0,255 expected true got false
  │ FAIL [expected PASS] WebGL test #8: should be 64,64,64,127\nat (0, 0) expected: 64,64,64,127 was 0,0,0,0
  │   → assert_true: should be 64,64,64,127
  │   → at (0, 0) expected: 64,64,64,127 was 0,0,0,0 expected true got false
  │ 
  │ reportTestResultsToHarness/<@http://web-platform.test:8000/_webgl/js/js-test-pre.js:123:24
  │ Test.prototype.step@http://web-platform.test:8000/resources/testharness.js:1958:25
  │ test@http://web-platform.test:8000/resources/testharness.js:548:30
  │ reportTestResultsToHarness@http://web-platform.test:8000/_webgl/js/js-test-pre.js:122:15
  │ testFailed@http://web-platform.test:8000/_webgl/js/js-test-pre.js:249:31
  │ checkCanvasRect/<@http://web-platform.test:8000/_webgl/js/webgl-test-utils.js:1269:19
  │ checkCanvasRectColor@http://web-platform.test:8000/_webgl/js/webgl-test-utils.js:1225:20
  │ checkCanvasRect@http://web-platform.test:8000/_webgl/js/webgl-test-utils.js:1257:23
  │ checkCanvas@http://web-platform.test:8000/_webgl/js/webgl-test-utils.js:1287:18
  │ testCopyTexImage2D@http://web-platform.test:8000/_webgl/conformance/textures/misc/copy-tex-image-2d-formats.html:188:7
  │ testFormats@http://web-platform.test:8000/_webgl/conformance/textures/misc/copy-tex-image-2d-formats.html:133:23
  └ @http://web-platform.test:8000/_webgl/conformance/textures/misc/copy-tex-image-2d-formats.html:91:12

We should adopt the same workround as FF.