Dirac internally allocates 5 images per plane and frame currently. One being the actual
image the other 4 being filtered for motion compensation.

Fixes: Out of memory
Fixes: 12870/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DIRAC_fuzzer-5684825871089664

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