The main buildscript expects 'print_backends' list to be defined.
Since printbackends is os_unix-only, we need to define this list
ourselves for other OSes.
https://bugzilla.gnome.org/show_bug.cgi?id=773299
This way, we can support external libraries providing implementations of
GtkMediaFile.
We also add a media backend called 'nomedia' that can be enabled to not
compile any support for GtkMediaFile. This is useful when people want to
statically compile GTK into an application that does not use media.
For now, this option is the default.
We also support a new environment variable GTK_MEDIA that allows
selecting the implementation to use.
GTK_MEDIA=help can be used to get info about the available
implementations.
We have to work around some ordering problems here. We still
manage to keep most of the guts in modules/input/meson.build,
so it's not too ugly overall.
(The autotools build solves this with a 'make -C ../../input/modules'
inside gtk/Makefile, but that's not something we can or want to do.)
