pdf2svg: migrate back to core
pdf2svg: migrate back to core

Turns out `pdf2svg` no longer depends on GTK+ or X11 (and previously did
this only by mistake).

Closes #97.

Signed-off-by: Xu Cheng <9a05244150b861e40c78843b801eed71bca54eac@me.com>

princeofdarkness76/homebrew-x11,BrewTestBot/homebrew-x11,UniqMartin/homebrew-x11