"TODO add the encoder here" - add platypus image encoder

[Kristine1975]

It has to exist somewhere, if only to test the decoder:

pico-extras/README.md

Line 23 in 77eae28

[platypus](src/common/platypus)| Decoder for a custom image compression format suitable for dithered images (good for RGB555) and suitable for decoding on RP2040 at scanline speeds ... i.e you can easily decode a 320x240 image 60x per second to avoid storing the uncompressed image for scanout video. It gets about 50% compression (but is designed only for 4x4 fixed dithered RGB555 images, so is somewhat specific!). TODO add the encoder here :-)

[Kristine1975]

This looks like the encoder (with some additional stuff for compressing several images in a row): https://github.com/raspberrypi/pico-playground/blob/eb6658e82069c603244b42d2bdd858a3fa270c63/apps/popcorn/converter/src/convert.cpp#L172