...which takes a raw filesystem pathname that is internally converted to a file
URL (similarly to what is supported for the INIFILENAME and URE_BOOTSTRAP
bootstrap variables in rtl::Bootstrap).  That way, the gtktiledviewer executable
doesn't need to try convert a pathname into a URL.

Also adapted various parameter names to make it obvious that URLs get passed,
not pathnames.

Change-Id: I33ab31fe142d94ee47885033ef48278ef5ff55a2
Reviewed-on: https://gerrit.libreoffice.org/24241Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>