wrlib: Improvement in the alpha channel support.

There are some problems in the alpha channel support, as is
reported at http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=72917

This patch add a new RCombineAlpha function, based on Gimp. This
function is called when needed in the raster.c functions.

This patch is based on the Brad Jorsch <anomie@users.sourceforge.net>
patch for the 0.62.1-0.1 version.

[crmafra: v1 was sent by Rodolfo kix Garcia <kix@kix.es>]

wrlib/Makefile.am

@@ -20,6 +20,7 @@ include_HEADERS = wraster.h
 
 libwraster_la_SOURCES = 	\
 	raster.c 	\
+	alpha_combine.c \
 	draw.c		\
 	color.c		\
 	load.c 		\