wayland: protect against unknown mimetype requests
wayland: protect against unknown mimetype requests

This oddly can be reproduced with weston+weston-dnd, when dragging
anything from GTK+ into weston-dnd, it will insist on picking its
custom application/x-wayland-dnd-flower mimetype, and this request
forwarded by the compositor, even if GTK+ didn't announce it on
its wl_data_source mimetype list. (What should probably happen here
is that the request is silenced, and/or weston-dnd picks (null))

This should be harmless, we are leaking though the fd in that case,
because the emission of GdkEventSelection on an unhandled mimetype
results in NOP. In order to avoid this, we should check whether the
mimetype is supported at all on the backend code and possibly close
the fd, this involves storing these in the first place.

https://bugzilla.gnome.org/show_bug.cgi?id=751414

alexlarsson/gtk,jigpu/gtk,jigpu/gtk,grubersjoe/adwaita,davidgumberg/gtk,davidgumberg/gtk,davidgumberg/gtk,grubersjoe/adwaita,Adamovskiy/gtk,chergert/gtk,grubersjoe/adwaita,alexlarsson/gtk,jadahl/gtk,davidgumberg/gtk,Adamovskiy/gtk,jadahl/gtk,chergert/gtk,Adamovskiy/gtk,alexlarsson/gtk,jigpu/gtk,chergert/gtk,Adamovskiy/gtk,alexlarsson/gtk,Adamovskiy/gtk,grubersjoe/adwaita,Adamovskiy/gtk,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,chergert/gtk,davidgumberg/gtk,davidgumberg/gtk,jigpu/gtk,jadahl/gtk,jigpu/gtk,jigpu/gtk,chergert/gtk,jadahl/gtk,jadahl/gtk,alexlarsson/gtk,davidgumberg/gtk,jadahl/gtk,chergert/gtk,davidgumberg/gtk,alexlarsson/gtk,jadahl/gtk,grubersjoe/adwaita,chergert/gtk,jigpu/gtk,grubersjoe/adwaita,Adamovskiy/gtk,chergert/gtk,Adamovskiy/gtk,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk