update treble boosting filter

[aitchFactor]

This is a filter that is designed to boost all the way up to Nyquist, unlike the previous one that rolled off at 15/16 of Nyquist. (This addresses part of #11.) Noting #5, the values have not been multiplied by 0.6, so filtered samples will be somewhat likely to clip and/or overflow, but they will not be inherently attenuated in any way.

[nyanpasu64]

idk if it feels too strong for me, it seems to make either high frequencies or quantization noise sound unpleasantly loud. perhaps picking one filter or the other should be an option (-g1/2)? will implement in https://github.com/nyanpasu64/BRRtools at some point.

yikes furnace's brr preview is unfiltered in the sample editor and filtered in the main window... furnace really is alpha-tier at snes