Make wayland load cursors on demand

Make wayland load cursors on demand

Copy just enough of libwayland-cursor to make our own
loading. This lets us drop the dependency on libwayland-cursor,
and changes the startup cost for cursor theme loading
from 25ms to 0.1ms.

At the same time, simplify the handling of scaled cursors -
instead of creating an array of theme objects, just make a
single theme object provide all scaled cursor sizes.