read AVI palette from the end of extradata
Official AVI specification says that stream header in case of video contains
BITMAPINFO, which is equal to BITMAPINFOHEADER and optional palette. Currently
lavf AVI demuxer thinks otherwise which produces garbage on codecs that have
both palette and extradata (luckily, there are not so many such codecs).
An example of such file is:
(IIRC, MSS1 or MSS2 also had such situation but they are still not supported
As a side note, passing palette in extradata as it's been done previously is
not quite correct since proper extra data is surplus bytes in
BITMAPINFOHEADER, not including palette.
Signed-off-by: Ronald S. Bultje <email@example.com>