x11: request available clipboard mime-types on video init

[redzimski]

Existing Issue(s)

https://github.com/libsdl-org/SDL/issues/12481

Note, I just copied the stuff from here. I don't know if that's the right way to fix this - I have not worked with X11 previously. But it seems to do the trick. https://github.com/libsdl-org/SDL/blob/6c61a94a4b6683ed7117685a21a5c8dbc1558fa9/src/video/x11/SDL_x11events.c#L1215-L1222