new option -capturedemo to capture a demo to an AVI file;

new option -capturedemo to capture a demo to an AVI file;
new cvars cl_capturevideo_width, cl_capturevideo_height to scale down video capture
when capturing >1GB, don't write an index chunk (in that case, an ix00/ix01 chunk has to be written, but that's not yet implemented)
when capturing, write a "dmlh" header to store the total number of frames, and store the partial number of frames in the "avih" header, as specified by OpenDML
when capturing, store the aspect ratio in a "vprp" header chunk (OpenDML)
make Host_Quit_f do nothing when already quitting (fixes "quit" issue while playing back a demo); note that Host_Quit_f is only used by the console and by -benchmark/-demo/-capturedemo

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@7321 d7cf8633-e32d-0410-b094-e92efae38249