allow differential gif anamation.

Reference: /n/sources/patch/applied/gifanamat
Date: Fri Dec  2 15:59:54 CET 2011
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/jpg/gif.c	Fri Dec  2 15:59:06 2011
+++ /sys/src/cmd/jpg/gif.c	Fri Dec  2 15:59:04 2011
@@ -14,6 +14,7 @@
 ulong	outchan = CMAP8;
 Image	**allims;
 Image	**allmasks;
+Rawimage	**allimages;
 int		which;
 int		defaultcolor = 1;
 
@@ -52,7 +53,10 @@
 	border(screen, r, -Border, nil, ZP);
 	r.min.x += allims[which]->r.min.x - allims[0]->r.min.x;
 	r.min.y += allims[which]->r.min.y - allims[0]->r.min.y;
-	drawop(screen, r, allims[which], allmasks[which], allims[which]->r.min, S);
+	if(which>0 && allimages[which]->gifflags&TRANSP)
+		drawop(screen, r, allims[which], allmasks[which], allims[which]->r.min, SoverD);
+	else
+		drawop(screen, r, allims[which], allmasks[which], allims[which]->r.min, S);
 	flushimage(display, 1);
 }
 
@@ -229,7 +233,7 @@
  * r is used only for reference; the image is already in c.
  */
 void
-whiteout(Rawimage *r, Rawimage *c)
+blackout(Rawimage *r, Rawimage *c)
 {
 	int i, trindex;
 	uchar *rp, *cp;
@@ -241,9 +245,9 @@
 		for(i=0; i<r->chanlen; i++){
 			if(*rp == trindex){
 				*cp++ = 0x00;
-				*cp++ = 0xFF;
-				*cp++ = 0xFF;
-				*cp++ = 0xFF;
+				*cp++ = 0x00;
+				*cp++ = 0x00;
+				*cp++ = 0x00;
 			}else{
 				*cp++ = 0xFF;
 				cp += 3;
@@ -254,7 +258,7 @@
 		for(i=0; i<r->chanlen; i++){
 			if(*rp == trindex){
 				*cp++ = 0x00;
-				*cp++ = 0xFF;
+				*cp++ = 0x00;
 			}else{
 				*cp++ = 0xFF;
 				cp++;
@@ -349,6 +353,7 @@
 		}
 	}
 
+	allimages = images;
 	allims = ims;
 	allmasks = masks;
 	loopcount = images[0]->gifloopcount;
@@ -380,7 +385,7 @@
 	if(nineflag){
 		if(images[0]->gifflags&TRANSP){
 			addalpha(rgbv[0]);
-			whiteout(images[0], rgbv[0]);
+			blackout(images[0], rgbv[0]);
 		}
 		chantostr(buf, outchan);
 		print("%11s %11d %11d %11d %11d ", buf,
@@ -392,7 +397,7 @@
 	}else if(cflag){
 		if(images[0]->gifflags&TRANSP){
 			addalpha(rgbv[0]);
-			whiteout(images[0], rgbv[0]);
+			blackout(images[0], rgbv[0]);
 		}
 		if(writerawimage(1, rgbv[0]) < 0){
 			fprint(2, "gif: %s: write error: %r\n", name);
@@ -403,6 +408,7 @@
     Return:
 	allims = nil;
 	allmasks = nil;
+	allimages = nil;
 	for(k=0; images[k]; k++){
 		for(j=0; j<images[k]->nchans; j++)
 			free(images[k]->chans[j]);