Add support for IMG's MSAA extension.

include/gpu/GrContext.h

Index: include/gpu/GrContext.h
===================================================================
--- include/gpu/GrContext.h	(revision 8223)
+++ include/gpu/GrContext.h	(working copy)
@@ -113,7 +113,7 @@
     size_t getGpuTextureCacheBytes() const;
 
     ///////////////////////////////////////////////////////////////////////////
-    // Textures
+    // Textures and Render Targets
 
     /**
      * Creates a new entry, based on the specified key and texture and returns it. The caller owns a
@@ -251,6 +251,12 @@
      */
     int getMaxTextureSize() const;
 
+    /**
+     * Returns the max sample count for a render target. It will be 0 if MSAA
+     * is not supported.
+     */
+    int getMaxSampleCount() const;
+
     ///////////////////////////////////////////////////////////////////////////
     // Render targets