The transparent_color field was never initialized. I _think_ this means
its value was always set to 0, or the blackest of blacks. Instead,
initialize it to the sentinel value, newly given the name
NO_TRANSPARENT_COLOR.
This exposed a second problem: The test for whether there was an existing
transparent color was wrong (backwards). I am guessing that this was not
found due to the first bug; since the converter had a transparent color,
the correct test would have led to always getting the error "Only one
color can be transparent at a time".
Closes#3723
* Initialize the EPaper display on the MagTag at start.
* Tweak the display send to take a const buffer.
* Correct Luma math
* Multiply the blue component, not add.
* Add all of the components together before dividing. This
reduces the impact of truncated division.
This also improves Palette so it stores the original RGB888 colors.
Lastly, it adds I2CDisplay as a display bus to talk over I2C. Particularly
useful for the SSD1306.
Fixes#1828. Fixes#1956
Also, renamed Sprite's palette to pixel_shader so it can be
anything that produces colors based on values (including color values).
Added a ColorConverter that converts RGB888 (found in bitmaps) to
RGB565 for the display.
Fixes#1182
