avm2: Always create four Stage3D instances #26968

Job	Run time
Paths filter	6s
Test Rust stable / ubuntu-24.04	8m 53s
Check dependencies	1m 24s
Test Rust stable / windows-latest	18m 2s
Test Rust stable / macos-14	5m 27s
Test Rust ${{ matrix.rust_version }} / ${{ matrix.os }}	0s
Test Rust nightly / ubuntu-24.04	8m 40s
Lints with Rust stable	4m 11s
Test Rust beta / ubuntu-24.04	9m 53s
Lints with Rust beta	4m 16s
Lints with Rust nightly	3m 51s
	1h 4m 43s

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