Epson HX-20 Emulator Improved Sound

Although I added support for the piezo speaker some time ago in an earlier version, it did not sound very nice, especially if trying to play back any music. The challenge is to try and feed the audio callback at an exact rate, but this has now been solved by letting the audio callback itself control the speed of the entire emulator. From now on, if piezo audio support is compiled in, the SDL…