Don't use nonportable (and redundant) GCC construct.

This doesn't exist in all versions of GCC, and the #else case generates
the same assembly code anyway with -O2.
video plugin should call VidExt_Quit when it is done
Avoid wrong colors in compressed fxt1 textures

mesa-7.8.1-24878-g3998cfa has better support for fxt1 and can even make the
YUV/ARGB hacks unnecessary. Mesa is now the upstream for this code and
therefore it is a good source when upgrading.
Allow to replace patented S3TC algorithm with compatible txc_dxtn

The S3TC is patented in some countries which makes it hard for distribution.
Mesa is using an external library which is using a compatible algorithm called
S2TC. But also S3TC implementations of this library are available.

Distributions can use this option to be on the safe side regarding the provided
functionality.