avcodec/dvbsubdec: Compute default CLUT based on bitmap analysis

Fixes displaying subtitles before any CLUT has been received
Fixes Ticket153

This will of course not display these initial subtitles in the correct
color (as that is not known at that point) but they should look clean
and not corrupted

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