plug-ins: load PDF without filling the background to white.

Some PDF have transparency on purpose. Filling with white, without any question,
is counter-productive and prevents from working on such PDF files.

Note that we could add an option with a checkbox, leaving one to decide whether
to fill the background or not. But it feels over-engineering the issue. The PDF
is transparent, so that's what we load, and that's it. If one wants to add a
background, it's easy to add a layer at the bottom of the stack (and they can
make it any color they want, not just white), after loading the file.

(cherry picked from commit c369827725)

plug-ins/common/file-pdf-load.c

@@ -984,10 +984,6 @@ render_page_to_surface (PopplerPage *page,
   poppler_page_render (page, cr);
   cairo_restore (cr);
 
-  cairo_set_operator (cr, CAIRO_OPERATOR_DEST_OVER);
-  cairo_set_source_rgb (cr, 1.0, 1.0, 1.0);
-  cairo_paint (cr);
-
   cairo_destroy (cr);
 
   return surface;