This is needed to ensure and up to date "wayland-scanner" is used,
as versions before 1.20.0 generate headers incompatible with
dynamic linking (WITH_GHOST_WAYLAND_DYNLOAD).
As the centos7 version of wayland is 1.15 so make this part of Blender's dependencies on Linux.
We intend to enable Wayland for Blender 3.4 release, this is needed for the build-bot.
Notes:
- The dependency is fairly small, 224K XZ file.
- https://gitlab.freedesktop.org/wayland/wayland/-/releases/1.21.0/downloads/wayland-1.21.0.tar.xz will have to be committed to https://svn.blender.org/svnroot/bf-blender/trunk/lib/packages/ separately.