Add OpenGL context to Viewer.

tools/viewer/sk_app/Window.h

 /*
 * Copyright 2016 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */
 
 #ifndef Window_DEFINED
 #define Window_DEFINED
 
@@ skipping 14 matching lines @@
     virtual ~Window() {};
 
     virtual void setTitle(const char*) = 0;
     virtual void show() = 0;
     virtual void inval() = 0;
 
     virtual bool scaleContentToFit() const { return false; }
     virtual bool supportsContentRect() const { return false; }
     virtual SkRect getContentRect() { return SkRect::MakeEmpty(); }
 
-    enum BackEndType {
+    enum BackendType {
         kNativeGL_BackendType,
-        kVulkan_BackendType
+        kVulkan_BackendType,
+
+        kLast_BackendType = kVulkan_BackendType
+    };
+    enum {
+        kBackendTypeCount = kLast_BackendType + 1
     };
 
-    virtual bool attach(BackEndType attachType,  const DisplayParams& params) = 0;
+    virtual bool attach(BackendType attachType,  const DisplayParams& params) = 0;
     void detach();
 
     // input handling
     enum class Key {
         kNONE,    //corresponds to android's UNKNOWN
 
         kLeftSoftKey,
         kRightSoftKey,
 
         kHome,    //!< the home key - added to match android
@@ skipping 95 matching lines @@
     OnMouseFunc  fMouseFunc;
     void*        fMouseUserData;
     OnPaintFunc  fPaintFunc;
     void*        fPaintUserData;
 
     WindowContext* fWindowContext;
 };
 
 }   // namespace sk_app
 #endif