swscale: fix an implementation-defined unsigned-to-signed conversion
swscale: fix some undefined signed left shifts

Based on a patch by Michael Niedermayer <michaelni@gmx.at>
swscale: Set alpha to opaque for internal palettes.

Fixes conversion of pal8 to rgb formats with alpha.

Updated references for 2 FATE tests which previously encoded fully
transparent images.

Based on a patch by Baptiste Coudurier <baptiste.coudurier@gmail.com>