Because re-generating the Skia image is slow and there is no point
storing both the image buffer and the SkiaImage in the cache as it
is basically the same information.
For embedded images the path is empty but we unconditionally used it to create a TextureCacheKey,
which clashes.
Instead, preserve and store the ImageCacheKey in the ImageInner variants.
This includes the cache of decoded images, the HTMLImage element support
and the SVG rendering adapter.
The objective is that Image holds an ImageInner, which is not a path
anymore that the backend has to process, but instead always either
decoded image data, a pointer to a static texture or an SVG tree that
can be rendered to the desired size.