Fix lighten_colour and double_lighten_colour to prevent alpha bleeding into red channel. Make macros clearer.

svn path=/trunk/netsurf/; revision=13742

desktop/plot_style.h

@@ -45,14 +45,14 @@
  * and adding a full quarter
  */
 #define lighten_colour(c1)						\
-	(((((3 * (c1 >> 16)) >> 2) + 64) << 16) |			\
+	(((((3 * ((c1 >> 16) & 0xff)) >> 2) + 64) << 16) |		\
 	 ((((3 * ((c1 >>  8) & 0xff)) >> 2) + 64) <<  8) |		\
 	 ((((3 * ( c1        & 0xff)) >> 2) + 64) <<  0))
 
 /* Lighten a colour by taking nine sixteenths of each channel's intensity and
  * adding a full intensity 7/16ths */
 #define double_lighten_colour(c1)					\
-	(((((9 * (c1 >> 16)) >> 4) + 112) << 16) |			\
+	(((((9 * ((c1 >> 16) & 0xff)) >> 4) + 112) << 16) |		\
 	 ((((9 * ((c1 >>  8) & 0xff)) >> 4) + 112) <<  8) |		\
 	 ((((9 * ( c1        & 0xff)) >> 4) + 112) <<  0))