The screen library brings the specific routines required by the e-paper screens.
Compared to other screens, the e-paper screens relies on a frame-buffer.
First, the commands for drawing text and graphics update the frame-buffer.
Then, the command
flush()sends the content of the frame-buffer to the screen and refreshes the panel.
The library deals with the logical screen, with
y coordinates, which takes into account the orientation.
It is an abstraction of the physical screen with horizontal and vertical coordinates defined by the data-sheet.
The screen library exposes the screen object.