SST 0.7 Beta (a.k.a. Somewhat Superior Technology)

This is a relatively small user-facing update, but there's a bunch of
internal code changes - and more still to come.

Actual features added in this version:

* FOV change support (fov_desired) now works on Portal 4104.

* Portal 4104's broken demo playback is also fixed; this persists after
  SST is unloaded, until the game is restarted.

And that's about it - the rest is all developer-facing. Refer to the
commit log for all that stuff.

Thanks, and have fun!