Support Ogg video

[devint1]

The Steam version of Descent 3 appears to distribute videos as Ogg rather than Interplay MVE:

~/.steam/steam/steamapps/common/Descent 3/movies $ ls -l 
total 189888
-rwxr-xr-x 1 devin devin 55086602 Jun 17  2023 end.ogv
-rwxr-xr-x 1 devin devin 63256483 Jun 17  2023 intro.ogv
-rwxr-xr-x 1 devin devin 48453990 Jun 17  2023 level1.ogv
-rwxr-xr-x 1 devin devin  7148789 Jun 17  2023 level5.ogv
-rwxr-xr-x 1 devin devin   335882 Jun 17  2023 mainmenu.ogv
-rwxr-xr-x 1 devin devin  1224781 Jun 17  2023 me.ogv
-rwxr-xr-x 1 devin devin 18910698 Jun 17  2023 mi.ogv
~/.steam/steam/steamapps/common/Descent 3/movies $ file intro.ogv 
intro.ogv: Ogg data, Theora video

It would be nice if this source distribution also supported this format.

[icculus]

(I'm assigning myself, since this PR is getting submitted eventually.)

[JeodC]

(I'm assigning myself, since this PR is getting submitted eventually.)

That's going to be a bit more than just you though, since we want to support both formats--and for sanity we'lll have to set a preference if both formats are present. Ew. I'm for it, but thinking ahead.