Move inval dedup to Window for wider usages.

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 63d5e19e5594716c9bc95742b0fac51fe8b9af75..4e21f2280e2d2458251d2253427e6dd3ea4ca695 100644
--- a/tools/viewer/sk_app/Window.h
+++ b/tools/viewer/sk_app/Window.h
@@ -27,7 +27,9 @@ public:
 
     virtual void setTitle(const char*) = 0;
     virtual void show() = 0;
-    virtual void inval() = 0;
+
+    // Check if the current window has any pending inval; call inval if not.
+    void invalIfNeeded();

[djsollen, 2016/05/23 16:48:51]

if a client should never call anything other than this function I would prefer
we just called it inval and then leave the existing call sites alone.  That
would mean that we need to change the protected method to onInval() but that
should require less modification than this current change.

 
     virtual bool scaleContentToFit() const { return false; }
     virtual bool supportsContentRect() const { return false; }
@@ -164,6 +166,16 @@ protected:
     void*        fPaintUserData;
 
     WindowContext* fWindowContext = nullptr;
+
+    // Make sure that either onPaint or markInvalReceived is called in the subclasses'
+    // implementations of inval. They unset fIsContentInvalided which allow invalIfNeeded
+    // to invalidate the content in the future.
+    virtual void inval() = 0;
+
+    // Uncheck fIsContentInvalided to allow future inval/onInval.
+    void markInvalProcessed();
+
+    bool fIsContentInvalidated = false;  // use this to avoid duplicate invalidate events
 };
 
 }   // namespace sk_app