port over cl_cin.cpp for SP

The file now almost completely matches MP (and ioquake3). SP now
starts on x64bit Linux. cl_ingameVideo was renamed to cl_inGameVideo
to match MP.

Still to port is the z_memman_pc diff, because in MP, we have the
Hunk_ functions, which are also in the ioquake3 engine, but they
are missing in SP. When we port these, CIN_UploadCinematic and
CIN_DrawCinematic will be able to match the MP version, and resampling
will work in CIN_UploadCinematic.