Convert image source files to SVGs

[gsnedders]

CSS2 has images in loads of different formats: *.eps, *.fig, *.gif, *.idraw, *.png, *.ps, and *.svg. (There's also *.jpe?g, but that's unused and removed in #2569.) The build system converts all of these into PNGs.

The *.eps, *.fig, *.idraw, and *.ps files should be converted into SVGs, rather than using proprietary or rare formats. The *.gifs should be converted into *.pngs and we should just drop the *.gifs entirely (having the gifs there and having the build system convert them is just… pointless).

Once we have all the images in SVGs, we can then ask a further question as to whether we should use the SVGs in the CSS2 spec.

And, uh, hey, I did this (also) on my flight to CA: master...gsnedders:css2-svg

Note this isn't yet landable: a lot of the *.pngs have a fair change from the current spec.