[Feature request] OPL4-wavetable support

[sdsnatcher]

The OPL4 (aka YMF278) was the last and most powerful sounchip of the OPL family. But now they seem to have been depleted and are becoming increasingly hard to find.

This feature request is then to add support for the OPL4-wavetable feature and the dual-DAC output. The FM block is exactly the same as the OPL3, but with a slightly different sample rate and is able to output to the 2nd DAC too.

Note: the 2nd DAC output was usually connected to a sound FX processor, like the YSS225.

References:
http://en.wikipedia.org/wiki/Yamaha_YMF278

PS: I'm aware that this isn't easy at all to do, so this request will probably receive a low priority. But it's good to keep it registered though.