saturate Size to 0 when subtracting two Sizes #4740

Job	Run time
format	16s
clippy-check	2m 30s
smithay-check-features	2m 45s
anvil-check-features	2m 26s
check-msrv	1m 25s
check-minimal	1m 21s
smallvil-check	1m 1s
smithay-tests	2m 48s
Documentation on Github Pages	1m 22s
WLCS: Core tests	1m 13s
WLCS: Output tests	1m 24s
WLCS: Pointer input tests	1m 19s
WLCS: Bad Buffer Test	1m 7s
	20m 57s

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