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/shared/webgpu/messages/recv.rs

@@ -8,8 +8,8 @@
 use arrayvec::ArrayVec;
 use base::id::PipelineId;
 use ipc_channel::ipc::{IpcSender, IpcSharedMemory};
+use pixels::IpcSnapshot;
 use serde::{Deserialize, Serialize};
-use snapshot::IpcSnapshot;
 use webrender_api::ImageKey;
 use webrender_api::units::DeviceIntSize;
 use wgpu_core::Label;