Support flicker-free booting

[Saviq]

With Ubuntu Core getting Plymouth for boot splash, we should have an option to (or even default to) using its buffer for the Frame background.

Things to consider:

• display mode changing between splash and Frame
• multi-output case
• transition to client app (e.g. fade in)

[Saviq]

This is now supported:

https://mir-server.io/docs/how-to-smooth-boot-with-ubuntu-frame