libhb: Fix decoding avi with palette

... and probably other formats that use a palette.
The palette is stored in AVPacket side data which we did not read.  So
read the side data and stash it in hb_buffer_t so that it can be used
later by the decoder.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6207 b64f7644-9d1e-0410-96f1-a4d463321fa5