Do not clear the source device in SkGpuDevice::drawDevice

src/gpu/SkGpuDevice.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkGpuDevice.h"
 
 #include "effects/GrBicubicEffect.h"
@@ skipping 1526 matching lines @@
     dstSize.fHeight = tmpDst.height();
 
     this->drawBitmapCommon(*draw, bitmap, &tmpSrc, &dstSize, paint, flags);
 }
 
 void SkGpuDevice::drawDevice(const SkDraw& draw, SkBaseDevice* device,
                              int x, int y, const SkPaint& paint) {
     // clear of the source device must occur before CHECK_SHOULD_DRAW
     GR_CREATE_TRACE_MARKER_CONTEXT("SkGpuDevice::drawDevice", fContext);
     SkGpuDevice* dev = static_cast<SkGpuDevice*>(device);
-    if (fNeedClear) {
-        // TODO: could check here whether we really need to draw at all
-        dev->clearAll();
-    }
+
+    // TODO: If the source device covers the whole of this device, we could
+    // omit fNeedsClear -related flushing.
+    // TODO: if source needs clear, we could maybe omit the draw fully.
 
     // drawDevice is defined to be in device coords.
     CHECK_SHOULD_DRAW(draw);
 
     GrRenderTarget* devRT = dev->accessRenderTarget();
     GrTexture* devTex;
     if (NULL == (devTex = devRT->asTexture())) {
         return;
     }
 
@@ skipping 344 matching lines @@
 #endif
 }
 
 SkImageFilter::Cache* SkGpuDevice::getImageFilterCache() {
     // We always return a transient cache, so it is freed after each
     // filter traversal.
     return SkImageFilter::Cache::Create(kDefaultImageFilterCacheSize);
 }
 
 #endif