Now SIMD works on amd64, although the build remains questionable,
because it assumes that all of its compiler flags will work.
This way we lose an uncomfortable git submodule.
Also, add Meson subprojects to .gitignore.
The gdk-pixbuf plugin does not work here, for whatever reason.
Moreover, close integration exposes higher bit depths, metadata,
and auxiliary images.
The library is awful and copylefted, but it's the only reasonable
thing that works.
Sadly, they don't have a canonical extension, and they don't show up
in the browser. We might want to employ some level of sniffing.
The first 16 bytes are enough to identify a lot.
We need to refactor, so that SVGs are pre-rendered on each change
of scaling by librsvg directly, because some elements may be rasterized.
It would be best to also support building against resvg.
