Async animated images need a single ImageKey which can point to any
frame represented by its own external image ID. Additionally a frame
could be referenced again directly (e.g. something shows/uses the first
frame of the animated image).
Before this patch, the ownership between an ImageKey and an external
image ID for a shared surface was not clearly expressed. This resulted
in a special command to release the reference to the external image
separately from deleting the image key.
This patch makes the strong reference to an external image ID and an
ImageKey directly related. Not only does this facilitate multiple
ImageKeys owning the same surface, it also simplifies the ownership
semantics.
Differential Revision: https://phabricator.services.mozilla.com/D7520
83a52f66de