Improve Virtual Boy emulator

[skyfloogle]

This PR updates the built-in Mednafen core with additional hardware bugs, as mentioned in #2442. It does not update Furnace's effect code to take these bugs into account. Key points worth keeping in mind for the exposed hardware effects:

• Envelopes will need to be reset by a write to SxINT somewhere between when one finishes and when the next one starts. Try making a module where an envelope goes all the way up or down, then try starting another one - it won't work.
• When an upward sweep goes beyond the maximum frequency, the channel cuts. It will need to be reactivated by a write to SxINT. This applies even if frequency modification is not enabled on that channel.
• When one round of modulation has happened, regardless of whether repeating is on, both non-repeating modulation and sweeping are blocked until the next write to SxINT.
• After writing to S5FQL or S5FQH, the effective frequency's low or high byte is locked to that register until literally anywhere else in audio RAM is written to, be it a register, wave RAM, or even unallocated space. I didn't test this one in Furnace, but it's worth keeping in mind.

[tildearrow]

Give me a couple days - I will get to this soon but right now I am focused in 0.6.8.2.