Add the ability to import and wrap external surfaces from external toolkits such as Qt and GTK.
Wayland surfaces and windows are more intrinsically tied to the client library than other windowing systems, so it is necessary to provide a way to initialize SDL with an existing wl_display object, which needs to be set prior to video system initialization, or export the internal SDL wl_display object for use by external applications or toolkits. For this, the global property SDL_PROPERTY_GLOBAL_VIDEO_WAYLAND_WL_DISPLAY_POINTER is used.
A Wayland example was added to testnative, and a basic example of Qt 6 interoperation is provided in the Wayland readme to demonstrate the use of external windows with both SDL owning the wl_display, and an external toolkit owning it.
Allow for the creation of SDL windows with a roleless surface that applications can use for their own purposes, such as with a windowing protocol other than XDG toplevel.
The property `wayland.surface_role_custom` will create a window with a surface that SDL can render to and handles input for, but is not associated with a toplevel window, so applications can use it for their own, custom purposes (e.g. wlr_layer_shell).
A test/minimal example is included in tests/testwaylandcustom.c
Wayland doesn't support programmatically setting the app icon, so note this restriction and specify that a desktop entry file that points to the desired icon image is required.
