Fix stereo signal processing within audiofilters.Filter

[relic-se]

Previously, left and right channels of stereo audio data were being treated as mono when processing through audiofilters.Filter. This update properly handles stereo data within this class by separating channel sample data into filter_states and filter_buffer.

For interest, @gamblor21 @todbot

Fixes #11024

[tannewt]

Are there any tests you could expand to test this?

[relic-se]

@tannewt Using the example code provided in #11024 (comment), here are the results of that test:

The stereo waveform graph above adheres to the following settings:

# Recording 1
FILTER = False
FILTER_BYPASS = True

# Recording 2
FILTER = True
FILTER_BYPASS = True

# Recording 3
FILTER = True
FILTER_BYPASS = False

Here's the same test but with varying filter frequencies:

Recording	Filter Frequency (hertz)
1	22050 (nyquist of sample rate)
2	1000
3	100

I can come up with a more objective test if desired. In my subjective opinion, it sounds significantly better in stereo and mono is unaffected.

[relic-se]

@tannewt The code I have provided does work, but I think there is room for optimization if you or someone else is willing to dive into it in a review.