Adding optimization to avoid image copy in SkSurface copy on write when content is discardable

include/core/SkSurface.h

Index: include/core/SkSurface.h
===================================================================
--- include/core/SkSurface.h	(revision 8710)
+++ include/core/SkSurface.h	(working copy)
@@ -79,10 +79,12 @@
     uint32_t generationID();
 
     /**
-     *  Call this if the contents have changed. This will (lazily) force a new
+     *  Call this if the contents are about to change. This will (lazily) force a new
      *  value to be returned from generationID() when it is called next.
+     *  @param canDiscardContents Should be true if upcoming changes will clear
+     *      or occlude prior contents, thus making them discardable.
      */

[reed1, 2013/04/18 14:48:33]

Perhaps an enum or bit-field, so at the call-site we don't have to
guess/remember what true/false mean?

-    void notifyContentChanged();
+    void notifyContentWillChange(bool canDiscardContents);
 
     /**
      *  Return a canvas that will draw into this surface. This will always