Add support for split display

Configure LEFT and RIGHT mixer separately
Framebuffer is already opened, this is not supposed to be here - look at https://github.com/Tasssadar/multirom/blob/master/framebuffer.c#L166 . In impl_open, you get pointer to the framebuffer struct, which has vi and fi structs with info from the framebuffer. Just call setDisplaySplit() from it.