Make the plain rust crate not link to gdk-pixbuf
With !904 (merged), the main crate doesn't use gdk-pixbuf anymore to load images. However, it still links to the gdk-pixbuf library because the main crate provides ImageSurface::to_pixbuf()
.
We should move that into the the C API crate, or something, so that "plain Rust" usage of librsvg doesn't require linking to gdk-pixbuf. It is the C API that requires GdkPixbuf
objects as output, and also the SVG pixbuf loader.