Fix #78923: Artifacts when convoluting image with transparency

We have to properly initialize `pxl` before using it.

Fix ported from <libgd/libgd#559>.

Author: willson-chen

NEWS

@@ -2,6 +2,10 @@ PHP                                                                        NEWS
 |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 ?? ??? ????, PHP 7.3.14
 
+- GD:
+  . Fixed bug #78923 (Artifacts when convoluting image with transparency).
+    (wilson chen)
+
 19 Dec 2019, PHP 7.3.13
 
 - Core:

ext/gd/libgd/gd_filter.c

@@ -264,6 +264,7 @@ int gdImageConvolution(gdImagePtr src, float filter[3][3], float filter_div, flo
 	for ( y=0; y<src->sy; y++) {
 		for(x=0; x<src->sx; x++) {
 			new_r = new_g = new_b = 0;
+			pxl = f(srcback, x, y);
 			new_a = gdImageAlpha(srcback, pxl);
 
 			for (j=0; j<3; j++) {

ext/gd/tests/bug47946.phpt

@@ -1,11 +1,9 @@
 --TEST--
 Bug #47946 (ImageConvolution overwrites background)
---DESCRIPTION--
-The expected image has black pixel artifacts, what is another issue, though
-(perhaps #40158).
 --SKIPIF--
 <?php
 if (!extension_loaded('gd')) die('skip gd extension not available');
+if (!GD_BUNDLED && version_compare(GD_VERSION, '2.2.5', '<=')) die('skip upstream fix not yet released');
 ?>
 --FILE--
 <?php