Adapt wayland-cursor API to use less WlBuffers

[elinorbgr]

The current API of wayland-cursor creates a wl_buffer (ondemand) for each image of the animation of every cursor. These buffers are kept alive for the whole lifetime of the app.

This is a bit wasteful in terms protocol resources, as it forces the server to keep the state of all these buffers in memory. In general it's good practice for wayland client to cleanup unused objects.

wayland-cursor should thus be changed to destroy the buffers when they receive a release() event from the server, and re-create them ondemand when they are needed.

This will probably require some changes to the the API of the crate though.