Until now, touch events sort of rely on XI_Enter/XI_Leave events accompanying
the pointer emulating touch in order to have a stage set on the device, These
events won't happen though if it's not a pointer emulating touch which happens
on the stage, causing touch events to be ignored.
Fix this by ensuring that the input device has a stage on XI_TouchBegin itself,
but only if it's not already set, so we don't possibly steal touch events to
an already interacting stage.
https://bugzilla.gnome.org/show_bug.cgi?id=732234
4c4e72a9dc