Add OSMesa (CDAT_BUILD_OFFSCREEN to enable).

@aashish24 @doutriaux1 

This adds the OSMesa OpenGL implementation for offscreen rendering in ParaView/VTK.

The dv3d tests were having issues, but these seem to be failing everywhere? Might need new baselines.

I tested CDAT_BUILD_MODE=ALL, and there were some vcs failures, but they went away when I bumped ParaView's VTK sha to match uvcdat-master. We should bump these once the contour label changes go in.