tools/target_dec_fuzzer: Increase maxpixels threshold for dirac

tools/target_dec_fuzzer: Increase maxpixels threshold for dirac

wavelets allow significant size expansion and they are also not
very fast.

Fixes: Timeout
Fixes: 16480/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DIRAC_fuzzer-5659892689403904 (108sec -> 17ms)
Fixes: 16480/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DIRAC_fuzzer-5749422717140992 (big ->15sec)
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Tomas Härdin <tjoppen@acc.umu.se>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>