Fix GH-17349: Tiled truecolor filling looses single color transparency

This is porting the relevant part of a previous upstream commit[1] to
align the behavior of our bundled libgd with upstream.  It should be
noted that this only works if the image actually has a transparent
color.

[1] <4770e2b2d5>

Closes GH-17351.

NEWS

@@ -5,6 +5,10 @@ PHP                                                                        NEWS
 - FTP:
   . Fixed bug GH-16800 (ftp functions can abort with EINTR). (nielsdos)
 
+- GD:
+  . Fixed bug GH-17349 (Tiled truecolor filling looses single color
+    transparency). (cmb)
+
 - Intl:
   . Fixed bug GH-11874 (intl causing segfault in docker images). (nielsdos)
 

ext/gd/libgd/gd.c

@@ -960,7 +960,9 @@ static int gdImageTileGet (gdImagePtr im, int x, int y)
 	srcy = y % gdImageSY(im->tile);
 	p = gdImageGetPixel(im->tile, srcx, srcy);
 
-	if (im->trueColor) {
+	if (p == im->tile->transparent) {
+		tileColor = im->transparent;
+	} else if (im->trueColor) {
 		if (im->tile->trueColor) {
 			tileColor = p;
 		} else {

ext/gd/tests/gh17349.phpt

@@ -0,0 +1,27 @@
+--TEST--
+GH-17349 (Tiled truecolor filling looses single color transparency)
+--EXTENSIONS--
+gd
+--FILE--
+<?php
+require_once __DIR__ . "/func.inc";
+
+$tile = imagecreatetruecolor(8, 8);
+$red = imagecolorallocate($tile, 255, 0, 0);
+imagefilledrectangle($tile, 0, 0, 7, 7, $red);
+imagecolortransparent($tile, $red);
+
+$im = imagecreatetruecolor(64, 64);
+$bg = imagecolorallocate($im, 255, 255, 255);
+imagefilledrectangle($im, 0, 0, 63, 63, $bg);
+imagecolortransparent($im, $bg);
+$fg = imagecolorallocate($im, 0, 0, 0);
+imageellipse($im, 31, 31, 50, 50, $fg);
+imagesettile($im, $tile);
+imagealphablending($im, false);
+imagefill($im, 31, 31, IMG_COLOR_TILED);
+
+test_image_equals_file(__DIR__ . "/gh17349.png", $im);
+?>
+--EXPECT--
+The images are equal.