wayland: Sanity check cursor image size

On Wayland it is a protocol violation to upload buffers with
dimensions that are not an integer multiple of the buffer scale.

Until recently, Mutter did not enforce this. When it started
doing so, some users started seeing crashes in GTK apps because the
cursor theme ended up with e.g. a 15x16 pixel image at scale of 2.

Add a small sanity check for this case.