class SetLayerImage

Associates an Image with the Layer.

The binding is legal in any composition mode, and is sticky: it survives composition

mode changes, and is replaced only by a later [`SetLayerImage`] call (or cleared by

[`ResetLayer`]). The image is displayed only while the layer's composition mode is

`IMAGE`.

The bound image must be large enough for the layer's stored `sample_rect`; this is

checked at [`fuchsia.ui.composition/Flatland.Present`], not here. See

[`LayerProperties.sample_rect`].

`layer_id` must refer to a valid, unreleased Layer.

`image_id` must refer to a valid, unreleased Image.

`acquire_fence` is an optional fence that must be signaled before the image is displayed.

`release_fence` is an optional fence, signaled by Scenic when *this binding* no

longer references the image. It is per-binding, not per-image: an image bound to

several layers has one such fence per binding, and the image is safe to write only

once every one of them has signaled.

A binding is ended by:

- being superseded by a later [`SetLayerImage`] on the same layer

- [`ResetLayer`]

- the layer ref-count reaching zero, either via [`ReleaseLayer`] or by being released

by a layer stack

- destruction of the session, which ends every binding it holds

For a binding ended while the session is alive, Scenic signals at the first frame

retire after that point. Bindings that never reached the screen (e.g. one superseded

within a single batch, or one lost when several

[`fuchsia.ui.composition/Flatland.Present`] calls are squashed into a single

frame) signal at that same retire rather than immediately. The signal is

therefore conservative by at most one frame; it is never early.