[epaper_spi] Enhance color capabilities for spectra e6 with dithering

[super-qua]

Component name

epaper_spi

Link to component documentation on our website

https://esphome.io/components/display/epaper_spi/

Describe the enhancement

Currently the implementation for mapping colors for the spectra e6 in https://github.com/esphome/esphome/blob/dev/esphome/components/epaper_spi/epaper_spi_spectra_e6.cpp limits the colors to the 6 built-in colors of the display.

By applying dithering techniques, this could be enhanced to display more colors, for example for LVGL widgets filling or for text colors.

In a simple form, limited to any combination of the 6 built-in colors, this could be managed by applying a checkerboard dithering between two built in colors, producing colors like cyan, magenta, orange, etc.

Example1: color magenta (rgb(255,0,255))
instead of returning RED for every pixel in a 3x3 area, return

R B R
B R B
R B R

(R=RED, B=BLUE) which produces a purple effect for the viewer.

Example2: lighter red color (rgb(255,130,130)) as a mix of RED and WHITE pixels

W R W
R W R
W R W

Note: This does not require more sophisticated dithering algorithms like FloydSteinberg, as in this simple form only a static mapping is applied. Therefore it's less compute-intensive.

By combining only two built-in colors, theoretically 15 more colors could be displayed on the epaper which would then result in 21 colors instead of 6.

Use cases

Allows more colors to be displayed on the epaper screen when drawing objects.

Anything else?

Blog post which explains the desired result in more detail: https://myembeddedstuff.com/e-ink-spectra-6-color

This is a continuation of the discussion in esphome/esphome#12709

[github-actions[bot]]

🏷️ I've automatically added the component: epaper_spi label to help categorize this feature request.