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.

With xcursor loading in-tree, we could also easily hook up loading cursors from resources, and actually guarantee the availability of the standard cursors. I'm just not sure how useful that is if it is limited to the wayland backend. Maybe the xcursor loading code should be moved to the frontend for that.