Properly draw the window background on window resize

Bug #7385090

This change gets rid of two silly asumptions:
- That a layer needs to be cleared with opaque black (it shouldn't,
it's already cleared to transparent and the view will cover it up
with its own background)
- The the clip should be dirty at the beginning of a frame only
when the render target is opaque

Change-Id: I415b6d3cab196057fb0281419a53fef601a44e28

Author: Romain Guy

core/java/android/view/ViewRootImpl.java

@@ -1424,8 +1424,6 @@ private void performTraversals() {
                             layerCanvas.setViewport(mWidth, mHeight);
                             layerCanvas.onPreDraw(null);
                             final int restoreCount = layerCanvas.save();
-                            
-                            layerCanvas.drawColor(0xff000000, PorterDuff.Mode.SRC);
 
                             int yoff;
                             final boolean scrolling = mScroller != null

libs/hwui/OpenGLRenderer.cpp

@@ -179,7 +179,7 @@ status_t OpenGLRenderer::prepareDirty(float left, float top, float right, float
     mSaveCount = 1;
 
     mSnapshot->setClip(left, top, right, bottom);
-    mDirtyClip = opaque;
+    mDirtyClip = true;
 
     updateLayers();