SDL2_mixer changed how it upsamples sounds, and some users complained
about the difference. Add an internal resampler that emulates how
SDL_mixer upsamples sounds. Since all Rebirth upsampling is an integer
upsample (11Khz -> 44KHz, 22KHz -> 44Khz), this internal emulation is
considerably simpler than a general purpose resampler.
With this commit, the builder can choose which resamplers to enable.
The available resamplers are chosen by preprocessor directive, and
presently do not have an SConstruct flag. For each resampler, if no
choice is made, then the resampler will be enabled if it is reasonable.
At least one of the resamplers must be enabled, or the build will fail
with a `#error` message.
The user may choose at program start time which of the available
resamplers to use for that execution of the program, through passing one
of the command line arguments:
- `-sdlmixer-resampler=sdl-native`
- `-sdlmixer-resampler=emulate-sdl1`
- `-sdlmixer-resampler=emulate-soundblaster16`
Runtime switching is not supported. If the user does not choose, then
the first enabled resampler from the list below will be used. The
available resamplers are:
- sdl_native (DXX_FEATURE_EXTERNAL_RESAMPLER_SDL_NATIVE) - delegates to
SDL_mixer / SDL2_mixer, the way Rebirth has historically done.
- emulate_sdl1 (DXX_FEATURE_INTERNAL_RESAMPLER_EMULATE_SDL1) - an
internal resampler that emulates how SDL_mixer worked. This should be
equivalent to sdl_native when using SDL_mixer, so by default it is
enabled when Rebirth is built to use SDL2_mixer and disabled when
Rebirth is built to use SDL_mixer. It can still be enabled manually
even when building for SDL_mixer, but this does not seem likely to be
useful.
- emulate_soundblaster16
(DXX_FEATURE_INTERNAL_RESAMPLER_EMULATE_SOUNDBLASTER16) - an internal
resampler submitted by @raptor in
5165efbc46. Some users reported audio
quality issues with this resampler, so it is not presently the
default.
