Corrected call to Gegl.init in Vala’s basic example.

	*  examples/vala/gegl-gtk-basic.vala (main): Pass the ref of args to
	   Gegl.init.
Escape $(datadir) in VAPIDIR and GIRDIR

Otherwise it will be evaluated to an empty string in configure. I am
not sure that we should actually be setting these values, but it does
allow gegl-gtk to find a prefixed gegl install automatically.
Use Babl’s cairo-ARGB32 format when blitting

This format has some optimizations and is noticeably faster than the
equivalent B'aG'aR'aA u8.

	*  gegl-gtk/internal/view-helper.c (view_helper_draw): Use babl’s
	   cairo-ARGB32 format when blitting the node.
Prevent segfault when setting autoscale policy without node.

View helper actually checks for null pointer, but it did *after* getting
its bounding box and that fails when the view has no node.

	*  gegl-gtk/internal/view-helper.c (update_autoscale): Get the
	   node's bounding box only if there is a valid node.