avm2: Make `TextField.getCharIndexAtPoint` accurate #26828

Job	Run time
Paths filter	6s
Test Node.js 22 / ubuntu-24.04	4m 14s
Test Node.js 22 / windows-latest	7m 48s
Test Node.js 23 / ubuntu-24.04	3m 57s
Test Node.js 23 / windows-latest	4m 47s
Build for browser tests	2m 3s
Test in ${{ matrix.browser }}	0s
Test Node.js ${{ matrix.node_version }} / ${{ matrix.os }}	0s
Test in chrome	1m 50s
Test in firefox	2m 51s
Test in edge	2m 12s
	29m 48s

Provide feedback