Downloading the subproject during the GTK build fails
in a flatpak build due to lack of network access.
flatpak-builder insists on having these things explicitly
spelled out as dependencies.
We are building against the 3.32 sdk, which has
a new-enough glib to default to the keyfile
settings backend.
Also, use fallback-x11, so we prefer Wayland.
Our flatpak-builder manifests include building Graphene from Git; since
we're building the GTK demos, it's pointless to build the Graphene tests
as well. Disabling tests and benchmarks avoids pointless installations
inside the Flatpak build repo that will just be removed by the time we
bundle the demo.
