GMs: change some GMs to use SkImage over SkBitmap

Also, add GetResourceAsImage() to Resources.h

Motivation:  test drawImage() as much as we test drawBitmap()

Review URL: https://codereview.chromium.org/1306133003
diff --git a/tools/Resources.h b/tools/Resources.h
index c9785a5..5e76eb3 100644
--- a/tools/Resources.h
+++ b/tools/Resources.h
@@ -11,6 +11,7 @@
 #include "SkString.h"
 
 class SkBitmap;
+class SkImage;
 class SkStreamAsset;
 class SkTypeface;
 
@@ -18,6 +19,7 @@
 void SetResourcePath(const char* );
 
 bool GetResourceAsBitmap(const char* resource, SkBitmap* dst);
+SkImage* GetResourceAsImage(const char* resource);
 SkStreamAsset* GetResourceAsStream(const char* resource);
 SkTypeface* GetResourceAsTypeface(const char* resource);