viewer: use internal texture to allow viewer to control texture as part of its

context

More intuitive code flow and allows the user to undock the viewer (which
forcibly destroys and recreates the context) and the viewer will handle
creation of the new texture in said new context.

app/render/backend/renderbackend.h

@@ -43,9 +43,9 @@ signals:
 protected:
   void RegenerateCacheID();
 
-  virtual bool InitInternal() = 0;
+  virtual bool InitInternal();
 
-  virtual void CloseInternal() = 0;
+  virtual void CloseInternal();
 
   virtual bool CompileInternal() = 0;