Add support for IMG's MSAA extension.

src/gpu/gl/GrGLInterface.cpp

Index: src/gpu/gl/GrGLInterface.cpp
===================================================================
--- src/gpu/gl/GrGLInterface.cpp	(revision 8240)
+++ src/gpu/gl/GrGLInterface.cpp	(working copy)
@@ -323,6 +323,12 @@
                 return false;
             }
         }
+        if (extensions.has("GL_IMG_multisampled_render_to_texture")) {
+            if (NULL == fRenderbufferStorageMultisample ||
+                NULL == fFramebufferTexture2DMultisample) {
+                return false;
+            }
+        }
     }
 
     // On ES buffer mapping is an extension. On Desktop