renderer: use shared backend when caching

I think there was an earlier commit with a similar name but turns out
I'd only done foundational work in that commit and never actually
properly set it up. Of course once I did, there were several issues that
needed fixing to make it work correctly, but now it works as expected.

Heavily optimizes larger projects by allowing cache jobs to only copy
what has changed.

app/render/backend/renderticket.cpp

@@ -22,7 +22,7 @@
 
 OLIVE_NAMESPACE_ENTER
 
-RenderTicket::RenderTicket(Type type, const TimeRange &time) :
+RenderTicket::RenderTicket(Type type, const QVariant &time) :
   finished_(false),
   cancelled_(false),
   time_(time),