Blender Git Loki

=Scons ffmpeg update and avi fix=

Updated scons to work with the ffmpeg binaries in
lib/windows/gcc again. Also fixed a bug reported by
Debolaz on irc. Blender's internal avi reading (which can
only read simple formats like motion jpeg) was telling the
video code it could read all avis, without checking their
codec.

To fix this, I copied AVI_open_movie(), and modified it to
to replace AVI_is_avi(). Now it properly checks the codec,
and validates the header.