fix extract image extracting from proxy producer instead of original producer
Review Request #128937 - Created Sept. 18, 2016 and submitted
When extract frame detects a proxy it tries to use the property "resource" for extracting from the original clip; however, this property points to the proxy clip, but not the original. Instead, the property "kdenlive:originalurl" looks like what originally was intended. Not least the preceeding comment mentions to use the original clip url ... and that strongly points towards the property "kdenlive:originalurl".
Using fresh recent Kdenlive git master.