Use Offscreen Surface for Split Screen

tools/viewer/sk_app/Window.h

Index: tools/viewer/sk_app/Window.h
diff --git a/tools/viewer/sk_app/Window.h b/tools/viewer/sk_app/Window.h
index 8b1378a80fdb6669e24102d8cbce7af2ef1f7f81..17777cf9a6daf736161e69c6d21111d199dbd6a9 100644
--- a/tools/viewer/sk_app/Window.h
+++ b/tools/viewer/sk_app/Window.h
@@ -15,6 +15,7 @@
 #include "SkJSONCPP.h"
 
 class SkCanvas;
+class SkSurface;
 
 namespace sk_app {
 
@@ -161,6 +162,9 @@ public:
     virtual const DisplayParams& getDisplayParams();
     void setDisplayParams(const DisplayParams& params);
 
+    // This is just for the sRGB split screen
+    sk_sp<SkSurface> getOffscreenSurface(bool sRGB);
+
 protected:
     Window();