Add a `GpuManager::device_mut()` method #4804

Job	Run time
format	7s
smithay-check-features	2m 43s
clippy-check	4m 4s
check-minimal	1m 7s
check-msrv	1m 8s
anvil-check-features	2m 13s
Documentation on Github Pages	1m 9s
smallvil-check	41s
smithay-tests	2m 25s
WLCS: Core tests	1m 10s
WLCS: Output tests	1m 17s
WLCS: Pointer input tests	1m 11s
WLCS: Bad Buffer Test	1m 8s
	20m 23s

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