Add write protection to OffsetPixelBuffer

No one should every try to write to this buffer. Enforce that by
throwing an exception if any one tries to get a writeable pointer
to the data.

common/rfb/EncodeManager.cxx

@@ -31,6 +31,7 @@
 #include <rfb/SMsgWriter.h>
 #include <rfb/UpdateTracker.h>
 #include <rfb/LogWriter.h>
+#include <rfb/Exception.h>
 
 #include <rfb/RawEncoder.h>
 #include <rfb/RREEncoder.h>
@@ -1494,6 +1495,11 @@ void EncodeManager::OffsetPixelBuffer::update(const PixelFormat& pf,
   stride = stride_;
 }
 
+rdr::U8* EncodeManager::OffsetPixelBuffer::getBufferRW(const Rect& r, int* stride)
+{
+  throw rfb::Exception("Invalid write attempt to OffsetPixelBuffer");
+}
+
 // Preprocessor generated, optimised methods
 
 #define BPP 8

common/rfb/EncodeManager.h

@@ -187,6 +187,9 @@ namespace rfb {
 
       void update(const PixelFormat& pf, int width, int height,
                   const rdr::U8* data_, int stride);
+
+    private:
+      virtual rdr::U8* getBufferRW(const Rect& r, int* stride);
     };
   };
 }