The "run once" initialisation of pixbuf modules shipped with gdk-pixbuf
itself would be skipped if an application was successfully calling
gdk_pixbuf_init_modules() first, as this would set the internal list of
file_formats to be non-NULL, and skip any initialisation of those
modules.

This fix makes sure that pixbuf modules shipped with gdk-pixbuf are
always initialised, regardless of whether gdk_pixbuf_init_modules()
successfully initialised an application provided one.

Fixes: fd1376b7

Fail hard when the XPM file's advertised width or height does not match
the data contained within the file.

In the same way that libXpm sanity checks it.

A laaaarge XPM file.

This is needed to build gtk+ and all dependencies from scratch on
Windows with meson subprojects.

For the development cycle.

gdk_pixbuf_init_modules() should be used by applications which
want to have gdk-pixbuf loader modules install in a separate
path, usually in the application's library directory.

https://bugzilla.gnome.org/show_bug.cgi?id=786035

This should allow applications to install gdk-pixbuf modules
in a separate directory from the system location.

https://bugzilla.gnome.org/show_bug.cgi?id=786035

Separate the builtin loaders setup from the external loaders
initialisation from the loaders.cache file.

https://bugzilla.gnome.org/show_bug.cgi?id=786035

For the new API to be added.

We're still using the deprecated form of passing gdk-pixbuf as a library
named argument, instead of the idiomatic positional argument, which lets
Meson figure out all the needed dependencies.

Bring it in line with GTK's and GLib's contribution guides, and make it
a bit more friendly and accessible for newcomers.