Add support for WebGL2 read and draw buffer settings

Adds support for the `ReadBuffer` and `DrawBuffers`
WebGL2 calls and the related parameter getters.

See:

- https://www.khronos.org/registry/webgl/specs/latest/2.0/#3.7.2
- https://www.khronos.org/registry/webgl/specs/latest/2.0/#3.7.4
- https://www.khronos.org/registry/webgl/specs/latest/2.0/#3.7.11

components/canvas/webgl_thread.rs

@@ -2024,6 +2024,8 @@ impl WebGLImpl {
                     attach(attachment)
                 }
             },
+            WebGLCommand::ReadBuffer(buffer) => gl.read_buffer(buffer),
+            WebGLCommand::DrawBuffers(ref buffers) => gl.draw_buffers(buffers),
         }
 
         // If debug asertions are enabled, then check the error state.