Use correct data types here, and gdImageSaveAlpha for the alpha flag

ext/gd/libgd/gd.c

@@ -3863,7 +3863,7 @@ int gdImageConvolution(gdImagePtr src, float filter[3][3], float filter_div, flo
 	int         x, y, i, j, new_a;
 	float       new_r, new_g, new_b;
 	int         new_pxl, pxl=0;
-	gdImagePtr  srcback, srctrans;
+	gdImagePtr  srcback;
 	typedef int (*FuncPtr)(gdImagePtr, int, int);
 	FuncPtr f;
 
@@ -3876,9 +3876,10 @@ int gdImageConvolution(gdImagePtr src, float filter[3][3], float filter_div, flo
 	if (srcback==NULL) {
 		return 0;
 	}
-	srcback->saveAlphaFlag = 1;
-	srctrans = gdImageColorAllocateAlpha(srcback, 0, 0, 0, 127);
-	gdImageFill(srcback, 0, 0, srctrans);
+
+	gdImageSaveAlpha(srcback, 1);
+	new_pxl = gdImageColorAllocateAlpha(srcback, 0, 0, 0, 127);
+	gdImageFill(srcback, 0, 0, new_pxl);
 
 	gdImageCopy(srcback, src,0,0,0,0,src->sx,src->sy);