Resolve imagecropauto() default $mode quirk

The `$mode` parameter of `imagecropauto()` defaults to `-1`.  However,
`-1` is changed to `GD_CROP_DEFAULT` right away, so basically the
default is `GD_CROP_DEFAULT`, which is rather confusing and
unnecessary.

Therefore, we change the default to `IMG_CROP_DEFAULT`, but still allow
an explicit `-1` to be passed for BC reasons, in which case we trigger
a deprecation notice, so we can rid the `-1` support eventually.

Author: cmb69

NEWS

@@ -19,6 +19,8 @@ PHP                                                                        NEWS
     pkg-config). (Eli Schwartz)
   . The bundled libgd behaves now like system libgd wrt. IMG_CROP_DEFAULT never
     falling back to IMG_CROP_SIDES.
+  . The default $mode parameter of imagecropauto() has been changed to
+    IMG_CROP_DEFAULT; passing -1 is now deprecated.
 
 - Hash:
   . The hash extension is now an integral part of PHP and cannot be disabled

UPGRADING

@@ -120,6 +120,8 @@ PHP 7.4 UPGRADE NOTES
     that of system libgd:
      * IMG_CROP_DEFAULT is no longer falling back to IMG_CROP_SIDES
      * Threshold-cropping now uses the algorithm of system libgd
+  . The default $mode parameter of imagecropauto() has been changed to
+    IMG_CROP_DEFAULT; passing -1 is now deprecated.
 
 - Hash:
   . The hash extension cannot be disabled anymore and is always an integral

ext/gd/gd.c

@@ -4685,12 +4685,12 @@ PHP_FUNCTION(imagecrop)
 }
 /* }}} */
 
-/* {{{ proto resource imagecropauto(resource im [, int mode [, float threshold [, int color]]])
+/* {{{ proto resource imagecropauto(resource im [, int mode = GD_CROP_DEFAULT [, float threshold [, int color]]])
    Crop an image automatically using one of the available modes. */
 PHP_FUNCTION(imagecropauto)
 {
 	zval *IM;
-	zend_long mode = -1;
+	zend_long mode = GD_CROP_DEFAULT;
 	zend_long color = -1;
 	double threshold = 0.5f;
 	gdImagePtr im;
@@ -4706,7 +4706,9 @@ PHP_FUNCTION(imagecropauto)
 
 	switch (mode) {
 		case -1:
+			php_error_docref(NULL, E_DEPRECATED, "Crop mode -1 is deprecated. Use IMG_CROP_DEFAULT instead.");
 			mode = GD_CROP_DEFAULT;
+			/* FALLTHRU */
 		case GD_CROP_DEFAULT:
 		case GD_CROP_TRANSPARENT:
 		case GD_CROP_BLACK: