Add OpenGL support to Linux viewer

tools/viewer/sk_app/Window.cpp

 /*
 * Copyright 2016 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */
 
 #include "Window.h"
 
 #include "SkSurface.h"
@@ skipping 57 matching lines @@
     if (backbuffer) {
         // draw into the canvas of this surface
         SkCanvas* canvas = backbuffer->getCanvas();
 
         fPaintFunc(canvas, fPaintUserData);
 
         canvas->flush();
 
         fWindowContext->swapBuffers();
     } else {
+        printf("no backbuffer!?\n");
         // try recreating testcontext
     }
 }
 
 void Window::onResize(uint32_t w, uint32_t h) {
     fWidth = w;
     fHeight = h;
     fWindowContext->resize(w, h);
 }
 
@@ skipping 10 matching lines @@
         fIsContentInvalidated = true;
         onInval();
     }
 }
 
 void Window::markInvalProcessed() {
     fIsContentInvalidated = false;
 }
 
 }   // namespace sk_app