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

tests/wpt/webgl/meta/conformance2/renderbuffers/readbuffer.html.ini

@@ -1,5 +1,5 @@
 [readbuffer.html]
-  expected: ERROR
+  expected: TIMEOUT
   [WebGL test #3: gl.getParameter(gl.READ_BUFFER) should be 1029 (of type number). Was null (of type object).]
     expected: FAIL