avcodec/gifenc: Only write frame palette entries that actually used

avcodec/gifenc: Only write frame palette entries that actually used

GIF palette entries are not compressed, and writing 256 entries,
which can be up to every frame, uses a significant amount of
space, especially in extreme cases, where palettes can be very
small.

Example, first six seconds of Tears of Steel, palette generated
with libimagequant, 320x240 resolution, and with transparency
optimization + per frame palette:

    * Before patch: 186765 bytes
    * After patch: 77895 bytes

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>