Create a proper interface base class for the viewport's framebuffer

This allows us to gracefully fall back to the FLTK code in case the
platform specific code cannot be used.
diff --git a/vncviewer/Viewport.h b/vncviewer/Viewport.h
index 7859db6..bd17655 100644
--- a/vncviewer/Viewport.h
+++ b/vncviewer/Viewport.h
@@ -72,6 +72,8 @@
 
 private:
 
+  PlatformPixelBuffer* createFramebuffer(int w, int h);
+
   static void handleUpdateTimeout(void *data);
 
   static void handleClipboardChange(int source, void *data);