Alter layer hoisting to only hoist layers for one canvas at a time

src/gpu/GrLayerCache.h

Index: src/gpu/GrLayerCache.h
diff --git a/src/gpu/GrLayerCache.h b/src/gpu/GrLayerCache.h
index adf904f833b4688126ec24805000fb1f2fbe4283..58887d6405c9e6d820c23dba96a498f4ab4a9dc3 100644
--- a/src/gpu/GrLayerCache.h
+++ b/src/gpu/GrLayerCache.h
@@ -208,10 +208,20 @@ public:
                                      const SkMatrix& ctm,
                                      const SkPaint* paint);
 
-    // Inform the cache that layer's cached image is now required. 
-    // Return true if the layer must be re-rendered. Return false if the
-    // layer was found in the cache and can be reused.
-    bool lock(GrCachedLayer* layer, const GrTextureDesc& desc, bool dontAtlas);
+    // Attempt to place 'layer' in the atlas. Return true on success; false on failure.
+    // When true is returned, 'needsRendering' will indicate if the layer must be (re)drawn.
+    // Additionally, the GPU resources will be locked.
+    bool tryToAtlas(GrCachedLayer* layer, const GrTextureDesc& desc, bool* needsRendering);
+
+    // Attempt to lock the GPU resources required for a layer. Return true on success;
+    // false on failure. When true is returned 'needsRendering' will indicate if the
+    // layer must be (re)drawn.
+    // Note that atlased layers should already have been locked and rendered so only
+    // free floating layers will have 'needsRendering' set.
+    // Currently, this path always uses a new scratch texture for non-Atlased layers
+    // and (thus) doesn't cache anything. This can yield a lot of re-rendering.
+    // TODO: allow rediscovery of free-floating layers that are still in the resource cache.
+    bool lock(GrCachedLayer* layer, const GrTextureDesc& desc, bool* needsRendering);
 
     // addUse is just here to keep the API symmetric
     void addUse(GrCachedLayer* layer) { layer->addUse(); }
@@ -235,6 +245,10 @@ public:
     void writeLayersToDisk(const SkString& dirName);
 #endif
 
+    static bool PlausiblyAtlasable(int width, int height) {
+        return width <= kPlotWidth && height <= kPlotHeight;
+    }
+
 private:
     static const int kAtlasTextureWidth = 1024;
     static const int kAtlasTextureHeight = 1024;
@@ -282,10 +296,6 @@ private:
     // Remove all the layers (and unlock any resources) associated with 'pictureID'
     void purge(uint32_t pictureID);
 
-    static bool PlausiblyAtlasable(int width, int height) {
-        return width <= kPlotWidth && height <= kPlotHeight;
-    }
-
     void purgePlot(GrPlot* plot);
 
     // Try to find a purgeable plot and clear it out. Return true if a plot