simple video subtitles support, defined in external text files, added a couple of...

simple video subtitles support, defined in external text files, added a couple of cvars controlling subtitles (cl_video_*). Added video scaling (video will not be drawn fullscreen but less size and centered) and brightness cvar. Added videoplaying() builtin for menuvm and csqc. Changed isdemo() function to be shared between menuvm and csqc (since it is same). Subtitles uses NOTIFY font.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@10099 d7cf8633-e32d-0410-b094-e92efae38249
::stable-branch::merge=406539417c894954ef88ecc2fc28c26ad01bbba6