Make individual audio library compilation units buildable.
Only files containing at least some platform independent code have been
considered.
Add audio library assembly replacement stubs.
Make al_midi.c buildable.
Satisfy the linker.
Mostly stub out audio library functions.
Update the makefile and remove the audio library placeholder.
Remove proprietary GF1 header.
Add native sound effects driver stub to the sound library.
Register native sound effects driver.
Enable native mixing.
Prevent sound related busy waiting.
Add libSDL2 audio for sound effects.
Connect native sound effects library driver with audio output.
Prevent an infinite loop while searching for a free voice handle.
The old behavior is unclear.
Fix audio callback buffer handling.
Fix SDL2 audio format and volume setting.
Add input polling during sound effect waiting loop.
Implement assembly for 8 bit mono mixing.
Fix sounds being suppressed in stereo mode.
Add missing include.
Implement the remaining basic mixer functions.
Reverb is not yet functional.