Don't enforce palette conversion when writing GD images

The GD image format is able to handle truecolor images as of libgd 2.0.12
(<https://github.com/libgd/libgd/blob/gd-2.2.3/src/gd_gd.c#L31-L33>).
Therefore we don't need the potentially lossy and time consuming palette
conversion.

This way, imagegd() can also be used to export raw truecolor image data.

UPGRADING

@@ -95,6 +95,8 @@ PHP 7.2 UPGRADE NOTES
 - GD:
     . Removed --enable-gd-native-ttf configuration option which was unused as
       of PHP 5.5.0 anyway.
+    . imagegd() stores truecolor images as real truecolor images. Formerly, they
+      have been converted to palette.
 
 - Mbstring
   . mb_check_encoding() accepts array parameter. Both key and value

ext/gd/gd.c

@@ -2565,9 +2565,6 @@ static void _php_image_output(INTERNAL_FUNCTION_PARAMETERS, int image_type, char
 				(*func_p)(im, i, fp);
 				break;
 			case PHP_GDIMG_TYPE_GD:
-				if (im->trueColor){
-					gdImageTrueColorToPalette(im,1,256);
-				}
 				(*func_p)(im, fp);
 				break;
 			case PHP_GDIMG_TYPE_GD2:
@@ -2619,9 +2616,6 @@ static void _php_image_output(INTERNAL_FUNCTION_PARAMETERS, int image_type, char
 				(*func_p)(im, q, tmp);
 				break;
 			case PHP_GDIMG_TYPE_GD:
-				if (im->trueColor) {
-					gdImageTrueColorToPalette(im,1,256);
-				}
 				(*func_p)(im, tmp);
 				break;
 			case PHP_GDIMG_TYPE_GD2:

ext/gd/tests/imagegd_truecolor.phpt

@@ -0,0 +1,34 @@
+--TEST--
+imagegd() writes truecolor images without palette conversion
+--SKIPIF--
+<?php
+if (!extension_loaded('gd')) die('skip gd extension not available');
+?>
+--FILE--
+<?php
+require_once __DIR__ . DIRECTORY_SEPARATOR . 'func.inc';
+
+$im = imagecreatetruecolor(10, 10);
+$white = imagecolorallocate($im, 255, 255, 255);
+imagefilledrectangle($im, 0,0, 9,9, $white);
+$blue = imagecolorallocate($im, 0, 0, 255);
+imagefilledrectangle($im, 3,3, 6,6, $blue);
+
+ob_start();
+imagegd($im);
+$buffer = ob_get_clean();
+
+$header = unpack('nsignature/nwidth/nheight/Ctruecolor', $buffer);
+printf("signature: %d\n", $header['signature']);
+printf("truecolor: %d\n", $header['truecolor']);
+printf("size: %d\n", strlen($buffer));
+
+test_image_equals_file(__DIR__ . DIRECTORY_SEPARATOR . 'imagegd_truecolor.png', $im);
+?>
+===DONE===
+--EXPECT--
+signature: 65534
+truecolor: 1
+size: 411
+The images are equal.
+===DONE===