Mesa drivers currently aren't supported until we do a minor overhaul of the shader subsystem. Right now only proprietary drivers are working properly. Mesa has a bug in their drivers because of an OpenGL extension we're using, so when using mesa it either does not start up or does not display properly. This is a known issue, and though technically not our fault, we are going to work around the issue with the minor shader overhaul.
ETA on that overhaul however is unknown -- I am putting it as a higher priority though.
