Refuse to create a touch resource if we don't have the capability
(for misbehaving clients), and don't attempt to use touch data
structures that are not initialized.
The grabbing state is now checked for both pointer/touch devices
within the seat, and the grab start coordinates returned by
meta_wayland_seat_get_grab_info().
https://bugzilla.gnome.org/show_bug.cgi?id=733631
Clutter touch events are translated into events being sent down
the interface resource, with the exception of FRAME/CANCEL events,
which are handled directly via an evdev event filter.
The seat now announces invariably the WL_SEAT_CAPABILITY_TOUCH
capability, this should be eventually updated as devices come and
go.
The creation of MetaWaylandTouchSurface structs is dynamic, attached
to the lifetime of first/last touch on the client surface, and only
if the surface requests the wl_touch interface. MetaWaylandTouchInfo
structs are created to track individual touches, and are locked to
a single MetaWaylandTouchSurface (the implicit grab surface) determined
on CLUTTER_TOUCH_BEGIN.
https://bugzilla.gnome.org/show_bug.cgi?id=724442
