compose: Fix SVG scaling with librsvg 2.52

[heftig]

We should only scale the canvas in the legacy path, since the viewport
already takes care of scaling.

Also, while we're here:

• We should cairo_save before scaling, so that on error the
  cairo_restore doesn't leave the canvas in the scaled state.
• The SVG size is only used in the legacy path.
• We don't inspect the error so no need to have tmp_error.

Fixes: #375

[ximion]

Sorry for the delay, and thank you for providing a patch!
This change looks good (except for the weird commented-out code, but that was already there previously).