lavc/rawdec: Use AV_PIX_FMT_PAL8 for raw 1 bpp video in AVI

 From
https://msdn.microsoft.com/en-us/library/windows/desktop/dd318229%28v=vs.85%29.aspx

:

"If biCompression equals BI_RGB and the bitmap uses 8 bpp or less, the
bitmap has a color table immediatelly following the BITMAPINFOHEADER
structure. The color table consists of an array of RGBQUAD values. The
size of the array is given by the biClrUsed member. If biClrUsed is
zero, the array contains the maximum number of colors for the given
bitdepth; that is, 2^biBitCount colors."

Nothing about "monochrome" here. Unfortunately, pal8 to monow conversion
seems a bit flaky, but that's another story.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>