test-renderers: Delete `rend3` dependency and stub out `gltf-render`. #933

Job	Run time
lint (stable)	3m 44s
build (ubuntu, stable, locked, true)	4m 40s
lint (nightly)	4m 57s
deny	40s
build (windows, stable, locked, false)	4m 24s
lint (beta)	4m 13s
incremental-mutants	26s
build (macos, stable, locked, false)	4m 43s
build (ubuntu, nightly, locked, false)	4m 40s
miri	2m 17s
no_std	48s
build (ubuntu, beta, locked, false)	4m 40s
build (ubuntu, stable, latest, false)	4m 39s
build (ubuntu, stable, minimal, false)	1m 23s
build-web	1s
features	1s
fuzz	1s
deploy-web	1s
	46m 18s

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