Add regression test for bug #47946

This bug had already been fixed, but apparently there's no regression
test yet, so we add one.

Note that the expected image has black pixel artifacts, which are
another issue (perhaps bug #40158), and would have to be adressed
separately.

ext/gd/tests/bug47946.phpt

@@ -0,0 +1,51 @@
+--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');
+?>
+--FILE--
+<?php
+function array_flatten($array)
+{
+    $tempArray = array();
+
+    foreach ($array as $value) {
+        if (is_array($value)) {
+            $tempArray = array_merge($tempArray, array_flatten($value));
+        } else {
+            $tempArray[] = $value;
+        }
+    }
+
+    return $tempArray;
+}
+
+function makeFilter($resource, $matrix, $offset = 1.0)
+{
+    $divisor = array_sum(array_flatten($matrix));
+    if ($divisor == 0) {
+        $divisor = .01;
+    }
+    return imageconvolution($resource, $matrix, $divisor, $offset);
+}
+
+$edgeMatrix = array(array(1, 0, 1), array(0, 5, 0), array(1, 0, 1));
+
+$im = imagecreatetruecolor(40, 40);
+imagealphablending($im, false);
+imagefilledrectangle($im, 0, 0, 39, 39, 0x7fffffff);
+imagefilledellipse($im, 19, 19, 20, 20, 0x00ff00);
+imagesavealpha($im, true);
+makeFilter($im, $edgeMatrix);
+
+require_once __DIR__ . '/func.inc';
+test_image_equals_file(__DIR__ . '/bug47946_exp.png', $im);
+?>
+===DONE===
+--EXPECT--
+The images are equal.
+===DONE===