Add PNM support

[tannewt]

According to atmakers: One tip: add pnm format next. It’s easy to support and imagemagick converts everything to it

[matt-land]

There are 7 formats in the family, and the pycon group worked on p1-p6.
http://netpbm.sourceforge.net/doc/pnm.html
Getting help from Scott on the palette object.

[tannewt]

@matt-land What help do you need?

[matt-land]

Can you point to any doc on using the palette object? We got it working with mono and I think grayscale, but we were kinda guessing.

With grey, we kept a set of all observed pixel values as we seek over the file, so it works kind of like below

""" 
color_list = set()
for decoded_pixel in (reading of file):
   # push into the bitmap
   bitmap[x, y] = decoded_pixel
   # add to the color list 
   color_list.add(decoded_pixel)

...
# color_list is now set([33, 255, 127, 56]) 

palette = palette(len(color_list))
for count, color in enumerate(color_list):
    # Like this ?
    palette[count] = 0x{color_as_hex}{color_as_hex}{color_as_hex} 
    # or like this for greyscale values?
     palette[count] = color
return bitmap, palette

[matt-land]

I think the PR getting landed will look like:

• get working RGB implementation
• get reviewed, go back and make the optimizations
• wait for the other PR#1 to get into master so builds are working again
• fight with the linting

[tannewt]

Hrm, docs aren't very useful. :-) https://circuitpython.readthedocs.io/en/latest/shared-bindings/displayio/Palette.html

The easiest thing is to return None for the palette because the values are encoding the color. An optimization would be to do a pass to count the number of unique colors and use a smaller bitmap value if possible. I'd do that second though.

[matt-land]

https://github.com/adafruit/circuitpython/pull/1874/files
I made the docs better, but it seems like the blind leading the blind.

[matt-land]

OK I have a branch that works with formats P1-P6.
I'm not ready to open a PR, but should I get hit by a bus, here is where I stopped:
This branch:
https://github.com/crookedstorm/Adafruit_CircuitPython_ImageLoad/tree/feature-tests

This doc covers testing on the hardware with a Feature M4 Express and a Feature 320x240 touch screen:
https://github.com/crookedstorm/Adafruit_CircuitPython_ImageLoad/blob/feature-tests/docs/pbm_test_code.py