pcx-pixbuf-loader - PCX image support for gdk-pixbuf 2.x.

Tested with 2.32.3 and 2.40.0.

This is the last version of gdk-pixbuf/io-pcx.c from upstream git,
repackaged as an unbundled/standalone project.

Retrieved via:
$ git clone https://github.com/GNOME/gdk-pixbuf.git
$ cd gdk-pixbuf
$ git checkout 6211fd86a8c37d64037550473dcf12ec050e4705

The next commit that touched io-pcx.c was its removal. From the
git log:

commit c8c11b689966b61d098197cd6ffec6e900402db9
Author: Matthias Clasen <mclasen@redhat.com>
Date:   Wed Sep 23 21:26:52 2015 -0400

    Drop the pcx loader
    
    This is another rarely seen, ancient image format, and nobody
    is maintaining this loader inside gdk-pixbuf.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=721372

...however, some of us may still have a need for PCX support (e.g.
for retrogaming/retrocomputing). One way to get it would be to compile
the old git tree (or an old release) without installing it, and copy just
libpixbufloader-pcx.so to the loader directory. It will work fine... but
that's a lot of extra stuff to compile just to get one file.

The io-pcx.c found here is identical to upstream's. However I have
cheated by creating "config.h" and "gdk-pixbuf-private.h" from scratch,
with appropriate defines and includes for a standalone build.

Building:

$ mkdir build
$ cd build
$ meson ..
$ ninja
$ sudo ninja install # prefix with DESTDIR=whatever, if you need to
$ sudo update-gdk-pixbuf-loaders

License: io-pcx.c is LGPL (see COPYING), copyright (C) 2003 Josh A. Beam.
The other files (this README, meson.build, *.h) are also LGPL, copyright
(C) 2019 B. Watson <yalhcru@gmail.com>.
