(feat) Create an OS overlay screen

We (@klausdieter1 and @penguineer) are working on an overlay framebuffer that can be used by the epicardium processes to write information on the display.

The API will be unchanged except for:

• add a function to select the overlay FB after opening the display
• add a function to disable the the overlay FB (for fullscreen apps)
• add a function to get the overlay visibility (background process can pause when overlay is not visible)

We will create a second framebuffer for the overlay. The color 0x0000 will be interpreted as transparent. epic_disp_update will compose the app and the os framebuffer for the display.

Apps will not see any difference, except that there are more information on the screen.