Inverse video and sanitize input

Text can be displayed in inverse video by either using the Inverse functions or the Ex function and an additional boolean value.

Sanitize character input to avoid reading invalid memory and crashing.

Generalize for drivers.Displayer

generalize for drivers.Displayer

This doesn't actually depend on anything specific for the SSD1306, so make it
more general for anything that implements driver.Displayer. As this may also
require changing the colors used for different devices, expose them as global
variables.