ventana: Remember to free slide->areas
test/driver: Show longer backtraces in Valgrind warnings
ventana: Handle floating-point OverlapX/Y values
ventana: Read iScan metadata from the copy in the level 0 XML

Some slides use a slightly different schema for the XML document in the
first TIFF directory.
ventana: Line-wrap fix
ventana: Read level 0 XML, not label XML, during detection

Some slides use a slightly different schema for the XML document in the
first TIFF directory.
ventana: Move tile coordinate calculation into helper function