Add pango_shape_full()

Add pango_shape_full()

Variant of pango_shape() that takes the full paragraph text as input.
This is then passed in entirety to HarfBuzz, which would allow certain
cross-run interactions (in Arabic for example).

When combined with upcoming HarfBuzz 0.9.5+, this fixes:

Red Hat Bug 858736 - [Spanish] Stray dotted circle rendered
https://bugzilla.redhat.com/show_bug.cgi?id=858736

and partially:

Bug 313181 - color changes break arabic shaping
https://bugzilla.gnome.org/show_bug.cgi?id=313181