Add support for reading GIFs without colormap

Cf. <libgd/libgd@fc38677>.

Closes GH-17364.

Author: cmb69

NEWS

@@ -22,6 +22,8 @@ PHP                                                                        NEWS
     images). (cmb)
   . Ported fix for libgd 223 (gdImageRotateGeneric() does not properly
     interpolate). (cmb)
+  . Added support for reading GIFs without colormap to bundled libgd. (Andrew
+    Burley, cmb)
 
 - Intl:
   . Fixed bug GH-11874 (intl causing segfault in docker images). (nielsdos)

ext/gd/libgd/gd_gif_in.c

@@ -244,8 +244,10 @@ gdImagePtr gdImageCreateFromGifCtx(gdIOCtxPtr fd) /* {{{ */
 					BitSet(buf[8], INTERLACE), &ZeroDataBlock);
 		} else {
 			if (!haveGlobalColormap) {
-				gdImageDestroy(im);
-				return 0;
+				// Still a valid gif, apply simple default palette as per spec
+				ColorMap[CM_RED][1] = 0xff;
+				ColorMap[CM_GREEN][1] = 0xff;
+				ColorMap[CM_BLUE][1] = 0xff;
 			}
 			ReadImage(im, fd, width, height,
 						ColorMap,

ext/gd/tests/gif_nocolormaps.gif

@@ -0,0 +1,11 @@
+--TEST--
+A GIF without any Global or Local color tables is still decoded
+--EXTENSIONS--
+gd
+--FILE--
+<?php
+$im = imagecreatefromgif(__DIR__ . "/gif_nocolormaps.gif");
+var_dump($im instanceof GdImage);
+?>
+--EXPECT--
+bool(true)