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_traits/webgl.rs

@@ -546,6 +546,8 @@ pub enum WebGLCommand {
     InvalidateFramebuffer(u32, Vec<u32>),
     InvalidateSubFramebuffer(u32, Vec<u32>, i32, i32, i32, i32),
     FramebufferTextureLayer(u32, u32, Option<WebGLTextureId>, i32, i32),
+    ReadBuffer(u32),
+    DrawBuffers(Vec<u32>),
 }
 
 macro_rules! nonzero_type {