tools/target_dec_fuzzer: adjust pixel threshold for TRUEMOTION2, as it allows coding gigantic images on tiny input

Fixes: Timeout (137sec -> 6sec)
Fixes: 16090/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TRUEMOTION2_fuzzer-5674245178261504

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