widget: Fix gtk_widget_pick() on 3d-transformed widgets

widget: Fix gtk_widget_pick() on 3d-transformed widgets

Picking is done by drawing a line along the parent's z axis and picking
at the intersection with the child's z=0 plane.

However, the previous code was casting a ray along the child's z axis.

This patch actually transforms the line to pick into the target's
coordinate system and then computes the corrrect intersection with the
z=0 plane.

Using graphene_point3d_interpolate() to compute the final intersection
point is a bit of abuse of that function, but I found no better way in
Graphene to achieve the same thing.