allow VMU displays to be separate from the main RetroArch display

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Is your feature request related to a problem? Please describe. In RetroArch, when the VMU display is turned on it appears above the game screen

Describe the solution you'd like make the VMU displays separate displays that can be shown to the side of the game screen. It would be nice for example to have the main game screen to the left, and all four VMUs could be displayed on the right in a line, with the VMU screens having a configurable size (for example, each VMU screen could take up 1/4th of the height of the main screen)

example of main screen on the left with 4 VMUs on the right:

 _____________ __
|             |__|
|             |__|
|             |__|
|_____________|__|

Additional context The Nintendo DS core DeSmuME has configurable screen layouts under Core Options > Screen > Screen Layout