Update build scripts; Add floating HUD

I updated the build scripts a bit: I put all output files in ${CMAKE_BINARY_DIR}, including vrquake2.pk3, and also added code to check if vrquake2.pk3 is up to date to avoid regenerating it on every build.

I also added a "floating HUD" that stays on the horizon instead of being fixed to the face, making it much easier to use the console (it's hard to read text on the edge of the display).
I'll have to look at it closer, but it looks good to me so far.