WebGL vertex attribute tests on macOS are failing

[asajeffrey]

For example:

  ▶ Unexpected subtest result in /_webgl/conformance/attribs/gl-disabled-vertex-attrib.html:
  │ FAIL [expected PASS] WebGL test #15: should be green\nat (0, 0) expected: 0,255,0,255 was 255,255,255,255
  │   → assert_true: should be green
  │   → at (0, 0) expected: 0,255,0,255 was 255,255,255,255 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
  └ @http://web-platform.test:8000/_webgl/conformance/attribs/gl-disabled-vertex-attrib.html:92:7

[asajeffrey]

This looks like it's responsible for a CRASH:

  ▶ CRASH [expected OK] /_webgl/conformance/attribs/gl-vertex-attrib-unconsumed-out-of-bounds.html
  │ 
  │ Stack trace for thread "WebGL thread"
  │ stack backtrace:
  │    0: <servo::backtrace::Print as core::fmt::Debug>::fmt
  │    1: core::fmt::write
  │    2: std::io::Write::write_fmt
  │    3: servo::backtrace::print
  │    4: servo::install_crash_handler::handler
  │    5: _sigtramp
  │    6: gleRunVertexSubmitImmediate
  │    7: glDrawArrays_Exec
  │    8: canvas::webgl_thread::WebGLImpl::apply
  │    9: canvas::webgl_thread::WebGLThread::handle_msg
  │   10: canvas::webgl_thread::WebGLThread::process
  │   11: std::sys_common::backtrace::__rust_begin_short_backtrace
  │   12: core::ops::function::FnOnce::call_once{{vtable.shim}}
  │   13: <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
  │   14: std::sys::unix::thread::Thread::new::thread_start
  │   15: _pthread_body
  └   16: _pthread_start

[asajeffrey]

The workaround in FF: https://searchfox.org/mozilla-central/rev/f1f75f0a2d995241fbc454edfab4be1064c544e1/dom/canvas/WebGLContextDraw.cpp#972

[asajeffrey]

The bugzilla bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1545473