Use Offscreen Surface for Split Screen

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 "GrRenderTarget.h"
 #include "SkSurface.h"
 #include "SkCanvas.h"
 #include "VulkanWindowContext.h"
 
 namespace sk_app {
 
 static bool default_char_func(SkUnichar c, uint32_t modifiers, void* userData) {
     return false;
 }
 
@@ skipping 86 matching lines @@
     if (!fIsContentInvalidated) {
         fIsContentInvalidated = true;
         onInval();
     }
 }
 
 void Window::markInvalProcessed() {
     fIsContentInvalidated = false;
 }
 
+sk_sp<SkSurface> Window::getOffscreenSurface(bool sRGB) {
+    return fWindowContext->createRenderSurface(nullptr, 0, true, sRGB);
+}
+
 }   // namespace sk_app