tablet: restrict the cursor surface to one per tool

The initial approach was to allow one surface to be re-used between tools,
seats and even used together as wl_pointer cursor surface. This has a few
drawbacks, most of which are related to managing the surface correctly in the
compositor. For example, the same cursor surface could have two different
hotspots. Animated cursors should animate independently rather than update at
the same time.

Furthermore: a client cannot know when a surface will cease being used as a
cursor surface. The basic assumption of "after focus out" is an implementation
detail in the compositor and unless the client unsets the cursor it is not
guaranteed that the surface is released. This again makes sharing a surface
less obvious - you cannot know if the wl_pointer surface is still in use when
you set it for a new wp_tablet_tool.

Avoid these headaches (and push some of them to the client) by simply
restricting a wl_surface to be assigned to a single tool. For the 99% use case
where we have one tablet with two tools (pen + eraser) this means we merely
get two extra surfaces, and the two don't usually share the same cursor shape
anyway. If sharing is absolutely necessary, a client may still opt to share
the underlying wl_buffer.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Carlos Garnacho <carlosg@gnome.org>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>

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