DPC fixes and improvements

This build fixes a timing regression in DPC emulation that crept into the recent releases and can cause Pitfall II to crash. In addition, DPC audio quality is improved.

Installation:

Download update.bin to your SD card and launch the update from the menu. Don't switch off your 2600 until the screen says "UPDATE COMPLETE".

If you have a ST-LINK/V2 programmer, you can also flash firmware.bin / firmware.hex directly.

This release identifies as R.EDWARDS 12 in the menu.