Fix motion vector computation after #17688. #57659

Job	Run time
build (windows-latest)	13m 4s
check-doc	11m 24s
build (ubuntu-latest)	7m 52s
check-bevy-internal-imports	4s
check-missing-examples-in-docs	27s
build (macos-latest)	7m 27s
ci	3m 19s
miri	15m 46s
run-examples-macos-metal	5m 12s
toml	2m 1s
typos	8s
check-missing-features-in-docs	39s
markdownlint	1m 6s
check-compiles	0s
check-compiles-no-std	0s
build-wasm-atomics	1m 45s
build-wasm	1m 46s
msrv	1m 47s
	1h 13m 47s

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