Cursor: 8 -> 1 bit alpha downsampling improvement

Cursor: 8 -> 1 bit alpha downsampling improvement

Mouse cursors with 8 bit alpha were downsampled to 1-bit opacity maps by
turning alpha values of 255 into 1 and everything else into 0. This
means that mostly-opaque pixels ended up completely invisible.

This patch changes the behaviour so that only pixels with less than 50%
alpha (0-127) are treated as transparent when converted to 1-bit alpha.

This greatly improves the subjective appearance of anti-aliased mouse
cursors, such as those used by macOS, when using a front-end UI without
support for alpha-blended cursors, such as some VNC clients.

Signed-off-by: Phil Dennis-Jordan <phil@philjordan.eu>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20240624101040.82726-1-phil@philjordan.eu>