Update WR (removal of RGB8, which isn't supported on some hardware).

Instead, we convert any source RGB8 images into RGBx (with
an opaque alpha channel).

components/canvas/webgl_thread.rs

@@ -487,7 +487,7 @@ impl<VR: WebVRRenderHandler + 'static, OB: WebGLThreadObserver> WebGLThread<VR,
             width: size.width as u32,
             height: size.height as u32,
             stride: None,
-            format: if alpha { webrender_api::ImageFormat::BGRA8 } else { webrender_api::ImageFormat::RGB8 },
+            format: webrender_api::ImageFormat::BGRA8,
             offset: 0,
             is_opaque: !alpha,
         }