This was accidentally missed out of https://github.com/alexlarsson/gthree/pull/63, meaning that graphene would only be built as a subproject if the developer cloned it manually. Now Meson clones it automatically into subprojects/ if needed (depending on the -Dwrap_mode option).
I figure that it's developers rather than distros who will be building with graphene as a subproject, so the latest commit is checked out. You could change to use a stable tag if you prefer.
This was accidentally missed out of https://github.com/alexlarsson/gthree/pull/63, meaning that graphene would only be built as a subproject if the developer cloned it manually. Now Meson clones it automatically into subprojects/ if needed (depending on the -Dwrap_mode option).