How should I current limit a LED matrix?
A great way to solve this is to cheat like crazy.

A constant current driver or sink IC will make certain that each LED only gets exactly as much current as it needs. TI's TLC59284 (16-channel sink) or TLC5916 (8-channel sink) will work for basic needs. Take a look at the rest of their Signage/Linear products if you want features such as per-channel current tweaking or PWM dimming (up to 12 bit resolution!). And most of them will work all the way down to 3V supply.

And of course, other vendors will have their own selection of drivers, so take a look around.