GNOME’s Mutter variable rate refresh support closer to being merged

Variable rate refresh (VRR / FreeSync / Adaptive-Sync) support for GNOME's Mutter compositor is closer to being merged. The native back-end support for VRR that has been in development the past two years is no longer considered a work-in-progress and it's believed there are no longer any blocking issues that would prevent this code from landing.

Every modern compositor should support this.