Fix #78849: GD build broken with -D SIGNED_COMPARE_SLOW

Apparently, this has not been tested for a long time, and might be a
refactoring relict.  Anyhow, we have to pass the context to
`GIFNextPixel` as well.

Author: cmb69

NEWS

@@ -2,6 +2,9 @@ PHP                                                                        NEWS
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 ?? ??? 2019, PHP 7.2.26
 
+- GD:
+  . Fixed bug #78849 (GD build broken with -D SIGNED_COMPARE_SLOW). (cmb)
+
 - Intl:
   . Fixed bug #78804 (Segmentation fault in Locale::filterMatches). (Stas)
 

ext/gd/libgd/gd_gif_out.c

@@ -534,7 +534,7 @@ compress(int init_bits, gdIOCtxPtr outfile, gdImagePtr im, GifCtx *ctx)
     output( (code_int)ctx->ClearCode, ctx );
 
 #ifdef SIGNED_COMPARE_SLOW
-    while ( (c = GIFNextPixel( im )) != (unsigned) EOF ) {
+    while ( (c = GIFNextPixel( im, ctx )) != (unsigned) EOF ) {
 #else /*SIGNED_COMPARE_SLOW*/
     while ( (c = GIFNextPixel( im, ctx )) != EOF ) {  /* } */
 #endif /*SIGNED_COMPARE_SLOW*/