Implement audio filters post DAC on the mixer

[DaveTCode]

https://wiki.nesdev.com/w/index.php/APU_Mixer has a section with:

The NES hardware follows the DACs with a surprisingly involved circuit that adds several low-pass and high-pass filters:

A first-order high-pass filter at 90 Hz
Another first-order high-pass filter at 440 Hz
A first-order low-pass filter at 14 kHz

that I don't yet understand but will need to implement at some point