basetextoverlay: Use the extents rectangle for positioning

basetextoverlay: Use the extents rectangle for positioning

the extents rectangle is what you need to know to properly position
a buffer that has been rendered in a surface of the ink rectangle
size. This patch make the placement on par with the placement we had
before without having to over allocate.

This patch also enable placement for vertical rendering. Note that
the halginement, valighment and line-alignment default are set to
the previous default when this property is set. This is for backward
compatibility, you can change the value after setting vertical render.

https://bugzilla.gnome.org/show_bug.cgi?id=728636