avm2: Always create four Stage3D instances #26970

Job	Run time
Paths filter	5s
Test Rust stable / ubuntu-24.04	9m 11s
Test Rust ${{ matrix.rust_version }} / ${{ matrix.os }}	0s
Test Rust stable / windows-latest	17m 28s
Check dependencies	1m 25s
Test Rust stable / macos-14	6m 22s
Test Rust nightly / ubuntu-24.04	8m 43s
Lints with Rust stable	8m 54s
Test Rust beta / ubuntu-24.04	10m 29s
Lints with Rust beta	4m 55s
Lints with Rust nightly	6m 54s
	1h 14m 26s

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