Can now specify a colour that will not be plotted so some part of the
Sprite are "transparent" and the TFT backforund shows through. New
example added for demonstration.
Sprites (images) can now be created in RAM, written to with text and
graphics and rendered to screen quickly, this makes it easier to get
flicker frre screen updates.
ESP8266 can typically create up to a 160x128 sprite, and ESP32 about
200x200 pixels. 16 bit colour only at the moment, may soon implement 8
bit colour to reduce RAM.