dialogs: Respect MacOS Reduced Motion option

e13cc635 uses gtk-enable-animation to determine whether
the more dynamic motion animation should be used for the
About Dialog credits. However, gtk-enable-animation does
not take into account the MacOS preferences.
This patch adds a check for this value using NSWorkspace's
accessibilityDisplayShouldReduceMotion boolean. Since the
value is TRUE on MacOS if the user wants animations turned
off, the boolean is inverted.

app/dialogs/about-dialog.c

@@ -42,6 +42,10 @@
 
 #include "gimp-intl.h"
 
+#ifdef PLATFORM_OSX
+#import <Cocoa/Cocoa.h>
+#endif
+
 
 /* The first authors are the creators and maintainers, don't shuffle
  * them
@@ -137,6 +141,14 @@ about_dialog_create (Gimp           *gimp,
                     "gtk-enable-animations", &dialog.use_animation,
                     NULL);
 
+      /* gtk-enable-animation does not currently respect MacOS's Reduced Motion
+       * setting. This check can be removed once that support is added in GTK.
+       */
+#ifdef PLATFORM_OSX
+      dialog.use_animation =
+        ! ([[NSWorkspace sharedWorkspace] accessibilityDisplayShouldReduceMotion]);
+#endif
+
       pixbuf = about_dialog_load_logo ();
 
       copyright = g_strdup_printf (GIMP_COPYRIGHT, GIMP_GIT_LAST_COMMIT_YEAR);