Make animating opacity cheaper. (#13583)

No need to update the compositing bits all the time.

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

diff --git a/packages/flutter/lib/src/rendering/proxy_box.dart b/packages/flutter/lib/src/rendering/proxy_box.dart
index 0404171..5271c16 100644
--- a/packages/flutter/lib/src/rendering/proxy_box.dart
+++ b/packages/flutter/lib/src/rendering/proxy_box.dart
@@ -736,11 +736,15 @@
     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);
-    markNeedsCompositingBitsUpdate();
+    if (didNeedCompositing != alwaysNeedsCompositing)
+      markNeedsCompositingBitsUpdate();
     markNeedsPaint();
-    markNeedsSemanticsUpdate();
+    if (wasVisible != (_alpha != 0))
+      markNeedsSemanticsUpdate();
   }
 
   int _alpha;
@@ -1354,8 +1358,10 @@
     assert(value != null);
     if (elevation == value)
       return;
+    final bool didNeedCompositing = alwaysNeedsCompositing;
     _elevation = value;
-    markNeedsCompositingBitsUpdate();
+    if (didNeedCompositing != alwaysNeedsCompositing)
+      markNeedsCompositingBitsUpdate();
     markNeedsPaint();
   }