add SkImage::NewFromPicture and a GM to test it

include/core/SkImage.h

Index: include/core/SkImage.h
diff --git a/include/core/SkImage.h b/include/core/SkImage.h
index e89ffa5e59a9037a76e8739fbed6df71413e05a6..2a62135bdb303ccdfaa852707a50b082fa324b40 100644
--- a/include/core/SkImage.h
+++ b/include/core/SkImage.h
@@ -20,6 +20,7 @@ class SkCanvas;
 class SkColorTable;
 class SkImageGenerator;
 class SkPaint;
+class SkPicture;
 class SkString;
 class SkSurface;
 class SkSurfaceProps;
@@ -141,6 +142,11 @@ public:
                                            const SkISize yuvSizes[3],
                                            GrSurfaceOrigin);
 
+    static SkImage* NewFromPicture(const SkPicture*, const SkISize& dimensions,
+                                   const SkMatrix*, const SkPaint*);
+
+    ///////////////////////////////////////////////////////////////////////////////////////////////
+
     int width() const { return fWidth; }
     int height() const { return fHeight; }
     uint32_t uniqueID() const { return fUniqueID; }