Fix image decode thrashing when drawing a very large source image to a GPU canvas

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 1271 matching lines @@
 // Break 'bitmap' into several tiles to draw it since it has already
 // been determined to be too large to fit in VRAM
 void SkGpuDevice::drawTiledBitmap(const SkBitmap& bitmap,
                                   const SkRect& srcRect,
                                   const SkIRect& clippedSrcIRect,
                                   const GrTextureParams& params,
                                   const SkPaint& paint,
                                   SkCanvas::DrawBitmapRectFlags flags,
                                   int tileSize,
                                   bool bicubic) {
+    // The following pixel lock is technically redundant, but it is desirable
+    // to lock outside of the tile loop to prevent redecoding the whole image
+    // at each tile in cases where 'bitmap' holds an SkDiscardablePixelRef that
+    // is larger than the limit of the discardable memory pool.
+    SkAutoLockPixels alp(bitmap);
     SkRect clippedSrcRect = SkRect::Make(clippedSrcIRect);
 
     int nx = bitmap.width() / tileSize;
     int ny = bitmap.height() / tileSize;
     for (int x = 0; x <= nx; x++) {
         for (int y = 0; y <= ny; y++) {
             SkRect tileR;
             tileR.set(SkIntToScalar(x * tileSize),
                       SkIntToScalar(y * tileSize),
                       SkIntToScalar((x + 1) * tileSize),
@@ skipping 696 matching lines @@
             const GrCachedLayer* layer = fContext->getLayerCache()->findLayerOrCreate(picture, i);
 
             SkDebugf("%d (%d), ", i, layer->layerID());
         }
     }
     SkDebugf("\n");
 #endif
 
     return false;
 }