Change fillRect() to take a buffer instead of a pixel

There has been some confusion if fillRect() should accept a buffer
or a pixel. This can cause misrendering if your data is not in the
native endian order. A buffer makes more sense here though, and
is what most of the callers are already assuming, so change the
API to follow that.
diff --git a/common/rfb/tightDecode.h b/common/rfb/tightDecode.h
index fe18ce8..7a1a9a2 100644
--- a/common/rfb/tightDecode.h
+++ b/common/rfb/tightDecode.h
@@ -77,7 +77,7 @@
     } else {
       pix = is->READ_PIXEL();
     }
-    pb->fillRect(serverpf, r, pix);
+    pb->fillRect(serverpf, r, &pix);
     return;
   }