Offer splitting and merging video outputs

Some cores outputs video of multiple screens into one video buffer as the API doesn't offer a way to split them anyway. retro-gtk should offer a way to segregate screens so they can be rendered as the application wants to. The screen should adapt the video output's size, the screen ratio and the pointer inputs accordingly. A typical use case is a Nintendo DS emulator.

Some other cores handle stereoscopy by outputing the two views of a single screen side-by-side. retro-gtk should offer a way to merge these views into a single anaglyph screen, offering to set color pair but defaulting to the common red-cyan.