Representing screen layout of virtual systems

[YoshiRulz]

At the very least, we need a new service which accommodates multi-screen systems (perhaps as simple as a list of IVideoProviders).

• This includes DS/i, 3DS, Virtual Boy, and I think 1 other edge case we should be emulating atm that I've forgotten. Plus all our linked handhelds and maybe some of MAME's systems.
• In the future it could include DOOM, linked GBA, and the Dreamcast's VMU.

The service would need to facilitate at least the relative positioning of the screens.

melonDS and Encore (and VirtualBoyee) currently combine 2 framebuffers into 1 within the core, which I've complained about at length, including in #3772.

It would be nice if the solution we come up with also covered screen rotation, as is currently handled in melonDS/Encore with non-sync settings.

• This would presumably be useful for most handhelds.
• Titles like GBA WarioWare: Twisted! and WonderSwan Rhyme Rider Kerorican could benefit from arbitrary rotations, not just quarter-turns.
  • And for GBC Kirby Tilt 'n' Tumble or NDS Tony Hawk's Motion I guess it would have to be 3D?

see also my notes #124