build: Use graphene as a fallback subproject

This allows building and testing mutter with graphene
even without the unreleased version from master. It also
helps CI -- not only GitLab's CI, but also Continuous.

When graphene 1.10 is released and broadly distributed,
we can remove this.

!458
4 jobs for !458 with gbsneto/graphene in 1 minute and 37 seconds
detached
Status Job ID Name Coverage
  Review
passed #318961
check-commit-log

00:00:23

 
  Build
failed #318962
build-mutter

00:01:13

 
  Test
skipped #318964
can-build-gnome-shell
skipped #318963
test-mutter
 
Name Stage Failure
failed
build-mutter Build

[902/906] Generating Meta-4.gir with a custom command.
Package graphene-gobject-1.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `graphene-gobject-1.0.pc'
to the PKG_CONFIG_PATH environment variable
Package 'graphene-gobject-1.0', required by 'virtual:world', not found

ninja: build stopped: subcommand failed.
ERROR: Job failed: exit code 1