* Remove mesa18 and libosmesa
mesa18 was introduced in #19528 as a way to maintain the old
autotools build of mesa separate from the new meson build.
We could add a second build system to mesa, but since mesa18 has
been deprecated for a long time, we'll just remove it.
libosmesa was used to multiplex the gl provider between mesa18
and mesa, and is thus unecessary. Remove it to reduce complexity
in the graphical stack.
* Remove references to mesa18 and libosmesa
* vtk: rework dependency on gl and osmesa
* memsurfer: rework dependency on vtk
* visit: minimal fix to avoid having both osmesa and glx
9c88a48a73