Fix temporary decoder buffer sizes

Some of these were excessively large because of redundant factors
in the size calculation.
diff --git a/common/rfb/hextileDecode.h b/common/rfb/hextileDecode.h
index 7affa15..47006a0 100644
--- a/common/rfb/hextileDecode.h
+++ b/common/rfb/hextileDecode.h
@@ -44,7 +44,7 @@
   Rect t;
   PIXEL_T bg = 0;
   PIXEL_T fg = 0;
-  PIXEL_T buf[16 * 16 * 4];
+  PIXEL_T buf[16 * 16];
 
   for (t.tl.y = r.tl.y; t.tl.y < r.br.y; t.tl.y += 16) {