Simplify `NdcRect::center()`. #133

Job	Run time
miri	3m 10s
build (ubuntu, stable, locked, true)	1h 5m 36s
build (windows, stable, locked, false)	1h 26m 44s
build (macos, stable, locked, false)	48m 27s
build (ubuntu, nightly, locked, false)	36m 18s
build (ubuntu, beta, locked, false)	50m 17s
build (ubuntu, stable, latest, false)	37m 7s
build (ubuntu, stable, minimal, false)	38m 0s
fuzz	18m 19s
deploy	4s
	6h 24m 2s

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