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

src/gpu/SkGpuDevice.cpp

Index: src/gpu/SkGpuDevice.cpp
diff --git a/src/gpu/SkGpuDevice.cpp b/src/gpu/SkGpuDevice.cpp
index 4af16109a4b6df1481c65ce98306277f732ce5ca..60b4e7c40db9254a9d571562db1b03bf32453a88 100644
--- a/src/gpu/SkGpuDevice.cpp
+++ b/src/gpu/SkGpuDevice.cpp
@@ -1289,6 +1289,11 @@ void SkGpuDevice::drawTiledBitmap(const SkBitmap& bitmap,
                                   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;