Provide a better R/W base PixelBuffer class

Clearly separates the read API from the write API
and also from actual implementation.
diff --git a/vncviewer/DesktopWindow.h b/vncviewer/DesktopWindow.h
index 08a6652..83a8c76 100644
--- a/vncviewer/DesktopWindow.h
+++ b/vncviewer/DesktopWindow.h
@@ -55,7 +55,7 @@
   void copyRect(const rfb::Rect& r, int srcX, int srcY);
 
   rdr::U8* getBufferRW(const rfb::Rect& r, int* stride);
-  void damageRect(const rfb::Rect& r);
+  void commitBufferRW(const rfb::Rect& r);
 
   void resizeFramebuffer(int new_w, int new_h);