Add 'content_offset' and 'padding' style properties for buttons (#9661)

* Add padding and content_offset style properties to buttons

src/gui/guiButton.h

@@ -336,5 +336,7 @@ class GUIButton : public gui::IGUIButton
 	gui::IGUIStaticText *StaticText;
 
 	core::rect<s32> BgMiddle;
+	core::rect<s32> Padding;
+	core::vector2d<s32> ContentOffset;
 	// END PATCH
 };