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.
revert the all-platforms form of CD device stops workaround from r948/r949 ...
... apply a CD device stops workaround only for linux for now.
cd_sdl: add back the informative message (was accidentally reverted in r953.)
AllocBlock: cache index of last used lightmap, start search there

Can speed up map loading by multiple seconds on levels with a lot of lightmaps, at a cost of using slightly more lightmaps (about 5% more).

https://sourceforge.net/p/quakespasm/patches/20/
Image_LoadImage: read pixel data in 1K chunks instead of calling getc in a loop