avcodec/dvbsubdec: Compute scoretab without iterating over pixels multiple times in compute_default_clut()

Improves speed 102->2 sec
Testcase: 8655/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DVBSUB_fuzzer-6277869285146624
Fixes: Timeout

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>