Add atlased layer purging

src/gpu/GrLayerCache.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrLayerCache_DEFINED
 #define GrLayerCache_DEFINED
 
@@ skipping 123 matching lines @@
     // For atlased layers, 'fLocked' is true if the layer is in a plot and
     // actively required for rendering. If the layer is in a plot but not
     // actively required for rendering, then 'fLocked' is false. If the
     // layer isn't in a plot then is can never be locked.
     bool            fLocked;
 };
 
 // The GrLayerCache caches pre-computed saveLayers for later rendering.
 // Non-atlased layers are stored in their own GrTexture while the atlased
 // layers share a single GrTexture.
-// Unlike the GrFontCache, the GrTexture atlas only has one GrAtlas (for 8888)
-// and one GrPlot (for the entire atlas). As such, the GrLayerCache
-// roughly combines the functionality of the GrFontCache and GrTextStrike
-// classes.
+//
+// The Layer Cache uses four plots for atlasing. It attempts to devote a single plot to
+// each picture but does allow sharing if a particular picture requires more
+// space. This is accomplished by:
+//   If the picture already has plots
+//      check its current plots for space - if they do return the new location
+//   else (the picture is new or an existing picture which needs more space):
+//      if there is an empty plot devote it to the picture
+//      failing that find the emptiest plot and devote it to the picture
+//      failing that (the atlas is full):
+//          Loop through the plots that aren't currently used by the picture in LRU order
+//          If all the layers are unlocked:
+//              delete them all, clear the rectanizer and devote the plot to the picture
+//      failing that - indicate that the atlas is full
+// 
+// In order to purge a given picture:
+//   Each plot used by the picture is considered.
+//   If all the layers in the plot are unlocked then they are all removed and
+//      the plot's rectanizer is reset. This can result in some layers owned
+//      by other pictures being ejected!
+//   If not all the layers are unlocked then just the purged picture's layers
+//      are removed and the rectanizer isn't reset. This can lead to a lot
+//      of wasted space in the plot!
 class GrLayerCache {
 public:
     GrLayerCache(GrContext*);
     ~GrLayerCache();
 
     // As a cache, the GrLayerCache can be ordered to free up all its cached
     // elements by the GrContext
     void freeAll();
 
     GrCachedLayer* findLayer(const SkPicture* picture, int layerID);
@@ skipping 63 matching lines @@
     // Try to find a purgeable plot and clear it out. Return true if a plot
     // was purged; false otherwise.
     bool purgePlot();
 
     // for testing
     friend class TestingAccess;
     int numLayers() const { return fLayerHash.count(); }
 };
 
 #endif