Sets the gravity hint for the context. The hint parameter can be
one of the following constants:
#PANGO_GRAVITY_HINT_NATURAL-
Scripts will take their natural gravity based on the base gravity and the script. This is the default.
#PANGO_GRAVITY_HINT_STRONG-
Always use the base gravity set, regardless of the script.
#PANGO_GRAVITY_HINT_LINE-
For scripts not in their natural direction (e.g. Latin in East gravity), choose per-script gravity such
that every script respects the line progression. This means, Latin and Arabic will take opposite gravities
and both flow top-to-bottom for example.
The gravity hint is used in laying vertical text out, and
is only relevant if gravity of the context as returned by
pcontext:GetGravity() is set to #PANGO_GRAVITY_EAST
or #PANGO_GRAVITY_WEST.