Dummy video mode for timedemo

[Blzut3]

Was running tests on 486s and was while observing the performance differences between the various video cards that I have, was thinking it would be handy to have a dummy video mode for FastDoom. Obviously this is only useful in conjunction with timedemo (and would probably make sense to have it default to doing -timedemo demo1 or something if not specified), but it would be interesting to have a build which runs the renderer against a RAM buffer but skips sending it to VRAM. This would take the video card out of the performance equation and help with doing more apples to apples testing between motherboards.

If done, an interesting feature for this build could also be a -novideo option which skips the renderer altogether.