Add "Sound Quality" inside the options menu.

[drHyperion451]

This add a "Sound Quality" option that lets the user change between original quality and remastered quality setting by chnaging the sndspeed cvar. This solves #169

Also I think it's okay to add a third option named "Custom" if the user adds another value to sndspeed directy to the console, or maybe think about other options. Having a quick look to the code it seems it only checks if the value is 11025 or not, so it shouldn't really matter.

Maybe this is not a necessary change. I think it's a good QoL if I want to choose the KexQuake sound quality instead of the old one, but I leave this up to the maintainers.

Maybe it's good to add a "lock" option when the game is set to original, since this option is useless in this scenario. We should not lock sndspeed though because of mod support.

[L-P]

sndspeed sole purpose is to apply a low-pass filter on everything when sndspeed.value == 11025 && snd_mixspeed->value == 44100. It does nothing else.

I recommend removing the cvar and the low-pass (or at least make it optional behind another snd_lowpass) rather than adding a misleading option to the menu.

snd_mixspeed is the one controlling the SDL audio device output format.