Use point3d → vec3 utility function

Instead of calling graphene_vec3_init() with each member of the
graphene_point3d_t structure.
docs: Fix the introspection annotations

The `vertices` argument is an out parameter.
docs: Add missing symbols to the API reference
docs: Do not use TRUE

We return the C99 stdbool.h `true`, not the GLib `TRUE` macro.