wip for drawables

Idea:

1. in its mutable recording state, keep a table of drawables on the side, and store an index in the record list.

2. In "immediate-mode" draw, just call the clients drawable directly (need access to our private list to turn the stored index into a proc)

3. when we "snap", we replace the list of drawables with a list of (sub) pictures, and then during playback of the snapped picture, we invoke a private drawable which just calls "drawPicture" on the index'd subpicture.

Review URL: https://codereview.chromium.org/727363003
diff --git a/samplecode/SampleArc.cpp b/samplecode/SampleArc.cpp
index c9310d6..a44eeb5 100644
--- a/samplecode/SampleArc.cpp
+++ b/samplecode/SampleArc.cpp
@@ -72,6 +72,12 @@
             paint.setColor(SK_ColorBLUE);
             canvas->drawArc(fR, 0, fSweep, false, paint);
         }
+
+        SkRect onGetBounds() SK_OVERRIDE {
+            SkRect r(fR);
+            r.outset(2, 2);
+            return r;
+        }
     };
 
 public: