Add a way for backends to provide a frame submission deadline
Allow backends to provide a deadline for frame submission. This would be useful to allow frame scheduling to work better (especially with the Wayland/X11 backends).
- For DRM, there's no composition so the deadline would be the next vblank. Still need to figure out what the interaction with variable refresh rate is.
- For Wayland, this would require some protocol changes. presentation-time doesn't provide this information yet.