Fix crash when showing external send GUI
Fix 'Location window is blank' (#5538)

Call set_session() on the location window from ARDOUR_UI::set_session(),
and show_all() on the location window's LocationUI in set_session,
so that it displays the markers and ranges of the current session.
common spelling of infinity (lowercase as libc's printf)
Merge branch 'master' into cairocanvas
minor fixes for recent cairocanvas work so that it builds on OS X (64bit)