Vega Strike Code

Fixed ffmpeg/swscale detection issues.

Fix nasty buffer underflow in vsfilesystem.cpp

Bugfix: playtime query on streaming sources was broken:
OpenAL only reports the current buffer's play time, so in order to get accurate time stamps,
we must add the current buffer's start time to the reported time.

Now the tests report 27ms maximum drift: ain't that great?

move ffmpeg module specific advance flags back to ffmpeg module, GTK2 still needs this done too

Merging fix from safemode branch -r12759, forgot to cache a few variables

cmake wasn't caching a few variables that we needed to

merging to safemode branch -r12757, fixes to cmake FFMPEG detection and general ffmpeg related cmake cleanups

cmake cleanups (mostly ffmpeg related)

Merging -r12749 of FindFFMPEG.cmake from audio branch to safemode branch, fixes detection for distro installed headers and libs

Merging trunk -r12745