widget: Fix sizeof error in memset

vte.c: In function 'vte_terminal_reset':
vte.c:14135:16: warning: argument to 'sizeof' in 'memset' call is the same expression as the destination; did you mean to remove the addressof? [-Wsizeof-pointer-memaccess]
          sizeof(&pvt->selection_origin));
                 ^
vteapp: Disable transparency by default

This is actually a bug in the vte widget, which should only use bg_opacity
when bg_transparent is TRUE, but since this feature is deprecated anyway,
there's no point wasting any time fixing this in the widget.