gles: Define a `TextureShaderElement` #4662

Job	Run time
format	9s
clippy-check	1m 10s
smithay-check-features	1m 10s
smithay-tests	0s
anvil-check-features	0s
check-msrv	0s
check-minimal	0s
Documentation on Github Pages	0s
smallvil-check	0s
WLCS: ${{ matrix.job_name }}	0s
	2m 29s

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