Don't unload shaders when shutting down renderer unless it's restarting

[xycaleth]

GLSL shaders currently get unloaded every time the renderer is shut down. With so many shaders needing to be compiled, it takes a long time for them to be unloaded and then load them again. This should save significant time when loading maps, and changing maps.

The only case when they need to be unloaded is when the renderer itself is going to be reloaded (e.g. when /vid_restart is run).