film_grain: round subsampled width/height up

Fixes #183. Fixes use of uninitialized data in apply_to_row_uv with odd
width in clusterfuzz-testcase-minimized-dav1d_fuzzer-5684823666982912.
Credits to oss-fuzz.
12 jobs for master in 7 minutes and 8 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed build-debian #236578
debian amd64

00:00:32

passed build-debian-aarch64 #236582
aarch64 debian

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passed build-debian-aarch64-clang-5 #236583
aarch64 clang5 debian

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passed build-debian-static #236579
debian amd64

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passed build-debian-werror #236585
aarch64 debian

00:00:26

passed build-macos #236584
macos

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passed build-win32 #236580
win32

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passed build-win64 #236581
win64

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  Test
passed test-debian #236586
debian amd64

00:00:54

passed test-debian-asan #236587
debian amd64

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passed test-debian-msan #236588
debian aarch64

00:03:19

passed test-debian-ubsan #236589
debian amd64

00:01:26