film_grain: copy unmodified planes before applying noise
film_grain: copy unmodified planes before applying noise

Luma output plane is used during chroma film grain. Fixes an use of
uninitialized value in iclip/apply_to_row_uv with
clusterfuzz-testcase-minimized-dav1d_fuzzer_mt-5636143299690496. Credits
to oss-fuzz.

videolan/dav1d