server: Fix thumbnails for XBox 360
server: Fix thumbnails for XBox 360

If a video file needed transcoding for the XBox, it would not display a
thumbnail because the generated url was adding the /th/ subrequest on top of
the /tr/ request.

This caused a "no transcoder available for WMVHIGH_FULL.wmv" warning as well
as a failing thumbnail request.

Tarnyko/rygel,jku/rygel,GNOME/rygel,GNOME/rygel,jku/rygel,Tarnyko/rygel,Pelagicore/Media-Manager-Rygel-patched,GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,Pelagicore/Media-Manager-Rygel-patched