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/win/rfb_win32/DeviceFrameBuffer.cxx b/win/rfb_win32/DeviceFrameBuffer.cxx
index 0ad06e9..d075c25 100644
--- a/win/rfb_win32/DeviceFrameBuffer.cxx
+++ b/win/rfb_win32/DeviceFrameBuffer.cxx
@@ -220,6 +220,7 @@
           }
         }
       }
+      cursorBm.commitBufferRW(cursorBm.getRect());
     }
 
     // Finally invert the AND mask so it's suitable for RFB and pack it into