avm2: Make `TextField.getCharIndexAtPoint` accurate #26833

Job	Run time
Paths filter	8s
Lints with Rust stable	4m 25s
Test Rust stable / ubuntu-24.04	4m 18s
Lints with Rust beta	4m 32s
Check dependencies	1m 32s
Test Rust stable / windows-latest	10m 48s
Test Rust ${{ matrix.rust_version }} / ${{ matrix.os }}	0s
Lints with Rust nightly	4m 18s
Test Rust stable / macos-14	3m 19s
Test Rust nightly / ubuntu-24.04	8m 54s
Test Rust beta / ubuntu-24.04	4m 46s
	47m 0s

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