Add check in GrGpu createTexture to make sure we don't create multisampled textures.

BUG=skia:

Review URL: https://codereview.chromium.org/1138223003

src/gpu/GrGpu.cpp

@@ -76,6 +76,11 @@ GrTexture* GrGpu::createTexture(const GrSurfaceDesc& origDesc, bool budgeted,
         return NULL;
     }
 
+    // We currently not support multisampled textures
+    if (!isRT && desc.fSampleCnt > 0) {
+        return NULL;
+    }
+
     GrTexture *tex = NULL;
 
     if (isRT) {