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/Encoder.cxx b/common/rfb/Encoder.cxx
index 56b5f38..18b6680 100644
--- a/common/rfb/Encoder.cxx
+++ b/common/rfb/Encoder.cxx
@@ -38,13 +38,11 @@
                              const PixelFormat& pf, const rdr::U8* colour)
 {
   ManagedPixelBuffer buffer(pf, width, height);
-  Pixel pixel;
 
   Palette palette;
   rdr::U32 palcol;
 
-  pixel = pf.pixelFromBuffer(colour);
-  buffer.fillRect(buffer.getRect(), pixel);
+  buffer.fillRect(buffer.getRect(), colour);
 
   palcol = 0;
   memcpy(&palcol, colour, pf.bpp/8);