libdeflate is now a dependency

Cough, cough.

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.

Sync comment

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 a Makefile

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.