cssparser: Make gsk_renderer_consume_url() return a string

cssparser: Make gsk_renderer_consume_url() return a string

We don't want to return a GFile because GFile can't handle can't deal
with data: urls.
That makes the code a bit more complicated that doesn't deal with those
URLs, but it makes the other code actually work.

GtkCssImageUrl also now decodes data urls immediately instead of only at
the first load. So don't use data urls if you care about performance.