Allow the GL context to be mocked

[scothis]

Mocking the GL context is useful in order to integration test
applications that interact with the mapbox-gl-js APIs, but do not have
WebGL available. By mocking the GL context, gl-js will continue to
respond to method invocations and emit events normally. It will fetch
styles, sprites, glyphs and tiles normally.

To mock the GL context, set mapboxgl.config.MOCK_GL = true before
creating the map instance.

Issue: #2750

[scothis]

Confirmed to work with a real browser hosted on SauceLabs that does not support WebGL.

[lucaswoj]

As I said in #2750, I am open to this change only if we cannot find a way to mock GL using non-invasive techniques such as proxyquire (even if that requires a small refactor).

[scothis]

I was able to get this working directly inside Studio.

  mockGL() {
    const MockWebGLRenderingContext = require('webgl-mock/src/WebGLRenderingContext');
    const MapboxGLCanvas = require('mapbox-gl/js/util/canvas');
    const contexts = new WeakMap();

    MapboxGLCanvas.prototype.getWebGLContext = function() {
      var context = contexts.get(this);
      if (!context) {
        context = new MockWebGLRenderingContext(this.canvas);
        context.getExtension = function(extension) {
          // disable VAO
          if (extension === 'OES_vertex_array_object') {
              return undefined;
          }
          return MockWebGLRenderingContext.prototype.getExtension.apply(this, arguments);
        };
        contexts.set(this, context);
      }
      return context;
    };
  }