Run WebGL tests in CI

[pjcozzi]

As discussed with @mramato offline:

• Replace all read pixels expectations with a function that can have a no-op expectation when the tests are ran with a "no WebGL" flag, e.g.,

expect(scene.renderForSpecs()).toEqual([0, 0, 0, 255]);

becomes

scene.expectRenderForSpecs([0, 0, 0, 255]);

• Replace the object returned by getContext with a mock object with GL functions that are no-ops, function() {}, except for getExtension, which should return mocked objects for the extensions we care about.
  • Likewise, all gl.get* functions should be mocked to return reasonable values.
• Run the tests and fix things I forgot.

This should only take a few hours and will be more reliable than mesa.