libdeflate is now a dependency
Be more precise if the file size is wrong
Do not get bitten by fallthrough again
Add the -c switch

It allows to select a compression library, currently zlib and libdeflate
Adapt pngblank to newest version of lgpng
Explain how to install locally
Fix the install rule
Update markdownlint configuration
Import a LICENSE file
Remove useless definition.
Create a README separated from pngblank.md
Add pledge(stdio)

And remove the useless fclose() on stdout.
Include a man page

Install it with the Makefile and also generate a README using mandoc.
Add the -p option to generate indexed PNGs

It means generating a PLTE chunk.
Add forgotten definition of usage()
Rework and re-import lgpng.{c,h}

Only adds the writing bits and chunks actually used by pnginfo.
Rework the write_* procedures and chunk structs

Now each chunk holds its length, type and crc value as well as its data.
A generic `struct chunk` is available to create common interfaces.