Lightweight KMS plane library.

libliftoff eases the use of KMS planes from userspace without standing
in your way. Users create "virtual planes" called layers, set KMS
properties on them, and libliftoff will pick hardware planes for these
layers if possible.

Resources:

 Blog post introducing the project:
    https://emersion.fr/blog/2019/xdc2019-wrap-up/#libliftoff
 FOSDEM 2020 talk:
    https://fosdem.org/2020/schedule/event/kms_planes/

libliftoff is used in production by gamescope on Steam Deck devices,
and work is underway to integrate it with wlroots.
