pixels: Move Snapshot and related data structures to pixels (#37590)

1. The `shared` directory is for the "_traits" crates, which will likely
   be moved out of this directly at some point and renamed "_api". These
   crates expose the API of crates to avoid circular dependencies.
   `Snapshot` isn't really this.
2. `Snapshot` is essentially a specialied kind of `Image` so it makes
   sense that it is grouped with other image-related things in `pixels`.

Testing: This should not change any behavior so is covered by existing
tests.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/offscreencanvas.rs

@@ -7,7 +7,7 @@ use std::cell::Cell;
 use dom_struct::dom_struct;
 use euclid::default::Size2D;
 use js::rust::{HandleObject, HandleValue};
-use snapshot::Snapshot;
+use pixels::Snapshot;
 
 use crate::canvas_context::{CanvasContext, OffscreenRenderingContext};
 use crate::dom::bindings::cell::{DomRefCell, Ref};