Add atlased layer purging

src/gpu/GrLayerCache.h

Index: src/gpu/GrLayerCache.h
diff --git a/src/gpu/GrLayerCache.h b/src/gpu/GrLayerCache.h
index e66d3330a6f94d6c20fb263ea1397937a5e2456d..8c6872e13dfa42eadcc898015d4165b87a48ad51 100644
--- a/src/gpu/GrLayerCache.h
+++ b/src/gpu/GrLayerCache.h
@@ -141,10 +141,29 @@ private:
 // 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*);