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.
Speed matters here, and this makes us about 20 percent faster
at loading large directories.
Moreover, libpng's PNG_ALPHA_BROKEN is indeed broken.
Thumbnails have a fairly fixed format, so there are very few practical
corner cases that could have been missed.
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.
