This 2-in-1 PR started with the goal of support the Bangle.js 2
smartwatch with *no USB*.
* Adds "secure" DFU build support with a committed private key.
* Adds 3-bit color support with one dummy bit for the JDI memory display
* Allows nrf boards to have a board_background_task() run in RUN_BACKGROUND_TASK.
This is needed because the Bangle.js 2 uses the watchdog to reset.
* Renamed port_background_task() to port_background_tick() to indicate it
runs on tick, not RUN_BACKGROUND_TASK.
* Marks serial connected when the display terminal is inited. This means
that safe mode messages show up on the display.
ACep, 7-color epaper displays also pack 3 bits in 4. So, I added that
support as well.
* Adds 3-bit ACeP color support for 7-color e-paper displays. (Not
watch related but similar due to color depth.)
* Allows a refresh sequence instead of a single int command. The 7" ACeP
display requires a data byte for refresh.
* Adds optional delay after resetting the display. The ACeP displays
need this. (Probably to load LUTs from flash.)
* Adds a cleaning phase for ACeP displays before the real refresh.
For both:
* Add dither support to Palette.
* Palette no longer converts colors when set. Instead, it caches
converted colors at each index.
* ColorConverter now caches the last converted color. It should make
conversions faster for repeated colors (not dithering.)
Before, when an OnDiskBitmap was a paletted bitmap type, the palette
was internal to the OnDiskBitmap, and it internally performed the palette
conversion itself. When using with a tilegrid, a ColorConverter() object
always had to be passed.
Now, an OnDiskBitmap has a "pixel_shader" property. If the bitmap is
a paletted bitmap type, it is a (modifiable) Palette object. Otherwise,
it is a ColorConverter() object as before. This allows palette effects
to be applied to paletted OnDiskBitmaps.
Code that used to say:
```python
face = displayio.TileGrid(odb, pixel_shader=displayio.ColorConverter())
```
must be updated to say:
```python
face = displayio.TileGrid(odb, pixel_shader=odb.pixel_shader)
```
Compatible code for 6.x and 7.x can say
```python
face = displayio.TileGrid(odb, pixel_shader=getattr(odb, 'pixel_shader', ColorConverter())
```
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
