sfx lowpass filter patch

cvar changes:

The "sndspeed" cvar / "-sndspeed" command-line option now control whether the low-pass filter is applied. If it's set to 11025 you get the low-pass filter, otherwise it's not used.

New "snd_mixspeed" cvar (and the "-mixspeed" command-line option); these default to 44100 and just control the sample rate we request from SDL. Not archived.

New “snd_filterquality” cvar, value can be 1-5. Not archived. The “5” setting closely matches the Windows resampler, and the “1” setting closely matches the OS X resampler. The default depends on the OS, “5” is used on windows builds, otherwise “1”, because I wanted the sfx to sound the same as they do with 0.85.9 on each platform. TODO is checking if a setting other than 1 sounds closer to the system resampler on linux (though it probably depends on the distro).

The lowpass filter is only used for sndspeed=11025 and snd_mixspeed=44100, though these are the defaults.


git-svn-id: svn://svn.code.sf.net/p/quakespasm/code/trunk/quakespasm@952 af15c1b1-3010-417e-b628-4374ebc0bcbd