a11y: Make get_character_extents return G_MININT when widget is off-screen

Like get_extents, G_MININT should be returned instead of random values
when the widget is off-screen.

This exports gtk_widget_accessible_on_screen so widget implementations
can just use it for the check.

Closes #1137
4 jobs for get_character_extents in 18 minutes and 22 seconds (queued for 10 seconds)
latest
Status Job ID Name Coverage
  Build
passed fedora-x86_64 #42148

00:09:13

passed msys2-mingw32 #42149
win32

00:05:50

 
  Flatpak
passed flatpak:demo #42150

00:06:18

passed flatpak:widget-factory #42151

00:08:55