Use PixelBuffer objects as the interface for encoders and decoders

This avoid a lot of unnecessary middle men. This also pushes the
responsibility for pixel format conversion into the encoders and
decoders. The new bufferFromBuffer() is used for direct conversion,
rather than PixelTransformer/TransImageGetter.
diff --git a/common/rfb/HextileDecoder.h b/common/rfb/HextileDecoder.h
index 67dc791..ffc495e 100644
--- a/common/rfb/HextileDecoder.h
+++ b/common/rfb/HextileDecoder.h
@@ -26,7 +26,7 @@
   public:
     HextileDecoder(CConnection* conn);
     virtual ~HextileDecoder();
-    virtual void readRect(const Rect& r, CMsgHandler* handler);
+    virtual void readRect(const Rect& r, ModifiablePixelBuffer* pb);
   };
 }
 #endif