Add Pitch Register for Audio Playback

[craigthomas]

The XO Chip specification allows for audio samples to be played back by the emulator. In order to do this, the emulator needs a pitch register to be added. The pitch controls the playback rate of the 16-byte sample contained in the audio buffer. The playback rate in Hz is calculated as follows:

4000 * 2^((pitch-64) / 48)

The specification says that the initial playback rate should be 4000 Hz, thus pitch should be initially set to 64. The value in the pitch register should be set with the operation:

Fx3A

This will load the contents of register Vx into the pitch register.