fix resurface crash with png, svg canvases

Regressed in 3dca82a

This was originally added in
cfc6dfd, but I don't think that
assessment is right at least for the current bug: cairo_destroy,
called after the backend's SetWidth, still holds a reference
to the surface and calls the closure (presumably for leading
<svg> or something?).

I can't get it to happen consistently enough to write a test,
sadly.

Fixes #2520

Author: chearon

src/backend/PdfBackend.cc

@@ -22,6 +22,7 @@ cairo_surface_t* PdfBackend::ensureSurface() {
 
 void PdfBackend::destroySurface() {
   if (surface) {
+    cairo_surface_finish(surface);
     cairo_surface_destroy(surface);
     surface = nullptr;
     assert(_closure);

src/backend/SvgBackend.cc

@@ -26,6 +26,7 @@ cairo_surface_t* SvgBackend::ensureSurface() {
 
 void SvgBackend::destroySurface() {
   if (surface) {
+    cairo_surface_finish(surface);
     cairo_surface_destroy(surface);
     surface = nullptr;
     assert(_closure);