More cleanup in the Vulkan viewer

tools/vulkan/viewer/Slide.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 Slide_DEFINED
 #define Slide_DEFINED
 
 #include "SkRefCnt.h"
 #include "SkString.h"
 
 class SkCanvas;
+class SkAnimTimer;
 
 class Slide : public SkRefCnt {
 public:
     virtual ~Slide() {}
 
     virtual void draw(SkCanvas* canvas) = 0;
+    virtual bool animate(const SkAnimTimer&) { return false;  }
+    virtual void load() {}
+    virtual void unload() {}
+
     SkString getName() { return fName; }
 
 protected:
     SkString    fName;
 };
 
 
 #endif