Now that we no longer have the extra layer of ImageContainers providing
a superficial level of caching/reuse of existing blob recordings, we
need some way to share recordings. This part adds support to
SurfaceCache to store BlobSurfaceProvider objects.

This includes the specialized code for invalidating SVG images. In
particular this is useful for animated SVG images. In general we want to
avoid changing the image key whenever possible so that we avoid
reallocating the underlying buffers in the compositor process for the
rasterized blob images.

We also need to track the ImageIntRegion used by the recording. If a
caller only wants a slice of the SVG image, then we need to track this
differentiation in our cache entries. At this time, we don't allow
substitutes for entries with a region exclusion.

Differential Revision: https://phabricator.services.mozilla.com/D126603