Create a minimal python port of the node.js module `lv_img_conv`. Only
the currently in use color formats `CF_INDEXED_1_BIT` and
`CF_TRUE_COLOR_ALPHA` are implemented.

Output only as binary with format `ARGB8565_RBSWAP`.

This is enough to create the `resources-1.13.0.zip`.

Python3 implements "propper" "banker's rounding" by rounding to the nearest
even number. Javascript rounds to the nearest integer.
To have the same output as the original JavaScript implementation add a custom
rounding function, which does "school" rounding (to the nearest integer)

Update CMake file in `resources` folder to call `lv_img_conf.py` instead of
node module.

For docker-files install `python3-pil` package for `lv_img_conv.py` script.
And remove the `lv_img_conv` node installation.

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gen_img: special handling for python lv_img_conv script

Not needed on Linux systems, as the shebang of the python script is read
and used. But just to be sure use the python interpreter found by CMake.
Also helps if tried to run on Windows host.

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doc: buildAndProgram: remove node script lv_img_conv mention

Remove node script `lv_img_conv` mention and replace it for
runtime-depency `python3-pil` of python script `lv_img_conv.py`.