SVG: allow replacing and re-loading the SVG "doc" (#1116)

Without this, once you load an SVG for the first time,
you can never change it. This meant that you couldn't change
a buttton's icon, for example, at runtime, even using a script apply.

Now that works, at no cost too, since we track which SVG body/"doc"
has been loaded to ensure we're not re-loading it on every draw
(which was already there, it was just too strict).