Add an option to use a background image

[AlanGriffiths]

It is almost possible to set a background:

$ snap set ubuntu-frame config="
add-wayland-extensions=zwlr_layer_shell_v1
"
ubuntu-frame &
swaybg -i /usr/share/backgrounds/joshua-coleman-something-yellow.jpg

You'll see the default greyscale background, followed by the "something-yellow" image. But both the greyscale and the image can get repainted (e.g. after clicking on the image). The result being that the greyscale often replaces the image.

---

Notes on possible implementations:

We should stop painting the greyscale if a background image is provided.

1. And enable the required zwlr_layer_shell_v1 extension for specific snaps. (The provider snap should be configurable though, as it needs to contain the custom image.)
2. Or, we could bundle swaybg (or similar) in the Frame snap and use a content interface to allow users to provide an image.

[wmww]

We could also add another Mir layer for internal clients, below the layer shell background layer.

[tobias-grasse]

From a user perspective, option 2 seems most interesting. We have a custom Pi-based appliance with a custom boot splash (formerly psplash, currently testing Core22 Plymouth implementation) and have configured Frame's background to a full black screen. On older Pi models like 3B, it takes ~12 seconds from frame's daemon start until WPE is loaded, so an intermediate background image would provide nicer user experience.

[Saviq]

@tobias-grasse your use case is more about flicker-free boot, where Frame would take the buffer from Plymouth, and used that as its background. Something we have on our radar, too. I filed #121 to track.