lavfi/paletteuse: fix to support transparency

lavfi/paletteuse: fix to support transparency

This patch enables paletteuse to identify the transparency in incoming
video and tag transparent pixels on outgoing video with the correct
index from the palette.

This requires tracking the transparency index in the palette,
establishing an alpha threshold below which a pixel is considered
transparent and above which the pixel is considered opaque, and
additional changes to track the alpha value throughout the conversion
process.

This change is a partial fix for https://trac.ffmpeg.org/ticket/4443
However, animated GIFs are still output incorrectly due to a bug
in gif optimization which does not correctly handle transparency.

Signed-off-by: Clément Bœsch <u@pkh.me>