gm: add a flag to force perspective usage in all gms

gm/gm.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "gm.h"
 using namespace skiagm;
 
@@ skipping 58 matching lines @@
     SkASSERT(NULL != canvas);
     SkBaseDevice* device = canvas->getTopDevice();
     GrRenderTarget* renderTarget = device->accessRenderTarget();
     if (NULL != renderTarget) {
         return renderTarget->getContext();
     }
     return NULL;
 }
 #endif
 
+DEFINE_bool(forcePerspectiveMatrix, false, "Force a perspective matrix.");
+
 // need to explicitly declare this, or we get some weird infinite loop llist
 template GMRegistry* SkTRegistry<GM*(*)(void*)>::gHead;