x11: request available clipboard mime-types on video init

[krizej]

Existing Issue(s)

#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.

SDL/src/video/x11/SDL_x11events.c

Lines 1215 to 1222 in 6c61a94

	/* when here we're notified that the clipboard had an external change, we request the
	* available mime types by asking for a conversion to the TARGETS format. We should get a
	* SelectionNotify event later, and when treating these results, we will push a ClipboardUpdated
	* event
	*/
	X11_XConvertSelection(display, videodata->atoms.CLIPBOARD, videodata->atoms.TARGETS,
	videodata->atoms.SDL_FORMATS, GetWindow(_this), CurrentTime);