webgl: Ensure we don't generate gl errors in the paint task

The validations performed in the script side of this should always
prevent this from happening, and this will allow us to catch bugs like
the previous one.

components/canvas/webgl_paint_task.rs

@@ -169,6 +169,9 @@ impl WebGLPaintTask {
             CanvasWebGLMsg::DrawingBufferHeight(sender) =>
                 self.send_drawing_buffer_height(sender),
         }
+
+        // FIXME: Convert to `debug_assert!` once tests are run with debug assertions
+        assert!(gl::get_error() == gl::NO_ERROR);
     }
 
     /// Creates a new `WebGLPaintTask` and returns the out-of-process sender and the in-process