Make animating opacity cheaper. (#13583)

No need to update the compositing bits all the time.

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -736,10 +736,14 @@ class RenderOpacity extends RenderProxyBox {
     assert(value >= 0.0 && value <= 1.0);
     if (_opacity == value)
       return;
+    final bool didNeedCompositing = alwaysNeedsCompositing;
+    final bool wasVisible = _alpha != 0;
     _opacity = value;
     _alpha = _getAlphaFromOpacity(_opacity);
+    if (didNeedCompositing != alwaysNeedsCompositing)
       markNeedsCompositingBitsUpdate();
     markNeedsPaint();
+    if (wasVisible != (_alpha != 0))
       markNeedsSemanticsUpdate();
   }
 
@@ -1354,7 +1358,9 @@ class RenderPhysicalModel extends _RenderCustomClip<RRect> {
     assert(value != null);
     if (elevation == value)
       return;
+    final bool didNeedCompositing = alwaysNeedsCompositing;
     _elevation = value;
+    if (didNeedCompositing != alwaysNeedsCompositing)
       markNeedsCompositingBitsUpdate();
     markNeedsPaint();
   }