When loading cursors at scale, we expect the
cursor images to have a size of scale * size.
If we don't find such images, load them at their
unscaled size and scale them up ourselves.
Without this, cursors will appear in unexpected
sizes depending on scales and themes.
Related: #4746
When building for homebrew/linuxbrew on Ubuntu 16.04, memfd_create() is
not available and causes the build to fail.
This adds a proper check for the function.
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.
