Merge branch 'joamag/audio-tests' into 'master'

frontends/sdl/README.md

frontends/sdl/res/test/blargg/dmg_sound/01-registers.png

frontends/sdl/res/test/blargg/dmg_sound/02-len ctr.png

frontends/sdl/res/test/blargg/dmg_sound/03-trigger.png

frontends/sdl/src/main.rs

frontends/sdl/src/test.rs

res/roms/test/blargg/dmg_sound/01-registers.gb

res/roms/test/blargg/dmg_sound/02-len ctr.gb

res/roms/test/blargg/dmg_sound/03-trigger.gb

res/roms/test/blargg/dmg_sound/04-sweep.gb

res/roms/test/blargg/dmg_sound/05-sweep details.gb

res/roms/test/blargg/dmg_sound/06-overflow on trigger.gb

res/roms/test/blargg/dmg_sound/07-len sweep period sync.gb

res/roms/test/blargg/dmg_sound/08-len ctr during power.gb

res/roms/test/blargg/dmg_sound/09-wave read while on.gb

res/roms/test/blargg/dmg_sound/10-wave trigger while on.gb

res/roms/test/blargg/dmg_sound/11-regs after power.gb

res/roms/test/blargg/dmg_sound/12-wave write while on.gb

src/apu.rs

src/data.rs