taanab osmesa builds working again

Recently taanab ran out of disk space, causing builds to fail. I presume to save space the superbuild that provided osmesa was removed. This caused the osmesa builds to fail, obviously.

I restored a stripped down superbuild with only the ENABLE_osmesa option on. This provides the OSmesa library under /home/kitware/code/pvsb/build-mesa/install, fixing the osmesa build issue.