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/script/dom/webglrenderingcontext.rs

@@ -1510,6 +1510,11 @@ impl WebGLRenderingContext {
 
         // FIXME: https://github.com/servo/servo/issues/13710
     }
+
+    pub fn valid_color_attachment_enum(&self, attachment: u32) -> bool {
+        let last_slot = constants::COLOR_ATTACHMENT0 + self.limits().max_color_attachments - 1;
+        constants::COLOR_ATTACHMENT0 <= attachment && attachment <= last_slot
+    }
 }
 
 #[cfg(not(feature = "webgl_backtrace"))]