[anotherworld] Implement save states

[felipesanches]

MAME has got gamesave infrastructure that we can benefit from in addition to the game's native password system. This can be very useful for debugging as well, since it can save me time during emulator development.

[felipesanches]

I've got a partial implementation of save states working today. Code is being force-pushed to a single-commit frequently amended branch at https://github.com/felipesanches/mame/tree/anotherworld

[felipesanches]

This is what currently works regarding save states:

• sound-sample playback on the 4 channels
• virtual machine execution / program-counter / stack / vm variables
• video buffers / graphics rendering

Work in progress - works partially, but still incorrectly, may even crash the emulator:

• The music tracker data is not yet being saved correctly.