Representing screen layout of virtual systems

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.

• What about separate windows? #4018
  • And hey, what about XR? If you have binocular displays, you could do cool things like drawing an OSD or ApiHawk layer as a separate plane, or only in one eye.

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

• The framebuffers included in savestates will have the settings baked in, mentioned in https://github.com/TASEmulators/BizHawk/pull/4333#issuecomment-3251338067.

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

Start by reviewing the existing multi-screen and framebuffer handling in melonDS and Encore, along with issue #3772 and the linked discussion. The proposed design should account for relative screen positioning and screen rotation across the listed systems; the issue does not identify specific BizHawk files or tests.