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.
With ImageInner::StaticTextures (the variant) and StaticTextures (the
struct), we need to disambiguate for the generated enum contructor
functions. This is done by prefixing the enum variant with their name.
Fixes#909
Since we declare the struct in C++, we don't need to declare it in
rust for cbindgen anymore, as long as we expose the Size2D type from
euclid to the cbindgen_private namespace
