First pass at pre-rendering saveLayers for GPU

src/core/SkPictureStateTree.h

Index: src/core/SkPictureStateTree.h
===================================================================
--- src/core/SkPictureStateTree.h	(revision 14614)
+++ src/core/SkPictureStateTree.h	(working copy)
@@ -74,8 +74,29 @@
      */
     class Iterator {
     public:
-        /** Returns the next offset into the picture stream, or kDrawComplete if complete. */
+        /** Returns the next op offset needed to create the drawing state
+            required by the queued up draw operation or the offset of the queued 
+            up draw operation itself. In the latter case, the next draw operation
+            will move into the queued up slot.
+            It retuns kDrawComplete when done. 
+            TODO: this might be better named nextOp
+        */
         uint32_t nextDraw();
+        /** Peek at the currently queued up draw op's offset. Note that this can
+            be different then what 'nextDraw' would return b.c. it is
+            the offset of the next _draw_ op while 'nextDraw' can return
+            the offsets to saveLayer and clip ops while it is creating the proper 
+            drawing context for the queued up draw op.
+        */
+        uint32_t peekDraw();
+        /** Stop trying to create the drawing context for the currently queued 
+            up _draw_ operation and queue up the next one. This call returns
+            the offset of the skipped _draw_ operation. Obviously (since the
+            correct drawing context has not been established), the skipped
+            _draw_ operation should not be issued. Returns kDrawComplete if
+            the end of the draw operations is reached.
+         */
+        uint32_t skipDraw();
         static const uint32_t kDrawComplete = SK_MaxU32;
         Iterator() : fPlaybackMatrix(), fValid(false) { }
         bool isValid() const { return fValid; }
@@ -111,6 +132,8 @@
         // Whether or not this is a valid iterator (the default public constructor sets this false)
         bool fValid;
 
+        uint32_t finish();
+
         friend class SkPictureStateTree;
     };