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())
```
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
Jeff Epler
microDev
Bryan Siepert
Scott Shawcroft