Fix default board for smaller screens
When the default board is created, we assume a 1080x1920 screen with a full 12x21 grid. This is not the case if mirr.OS is set up on smaller screens. We need a slimmed-down default board for those screens, down to a 800x480 screen, like the 7" RasPi touch screen.
Resolves #761 #754
Edited by Tobias Grasse