Semantic changes to Wayland means that we cannot rely on the compositor
setting a pointer buffer for us if set it to nil. The first part of fixing
this is to create an shm buffer containing the bytes for our cursor.
The best way to do this currently is to load the cursor from the well known
location where weston instals its cursor images. The code to implemente this
was derivedlifted from the Wayland backend in GTK+.
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