curve: Add gsk_curve_get_normal

Its easy but thats no reason not to have this api.

gsk/gskcurve.c

@@ -1625,6 +1625,19 @@ gsk_curve_get_tangent (const GskCurve   *curve,
   get_class (curve->op)->get_tangent (curve, progress, tangent);
 }
 
+void
+gsk_curve_get_normal (const GskCurve  *curve,
+                      float            t,
+                      graphene_vec2_t *normal)
+{
+  graphene_vec2_t tangent;
+
+  gsk_curve_get_tangent (curve, t, &tangent);
+  graphene_vec2_init (normal,
+                      - graphene_vec2_get_y (&tangent),
+                      graphene_vec2_get_x (&tangent));
+}
+
 void
 gsk_curve_split (const GskCurve *curve,
                  float           progress,

gsk/gskcurveprivate.h

@@ -123,6 +123,9 @@ void                    gsk_curve_get_point                     (const GskCurve
 void                    gsk_curve_get_tangent                   (const GskCurve         *curve,
                                                                  float                   progress,
                                                                  graphene_vec2_t        *tangent);
+void                    gsk_curve_get_normal                    (const GskCurve         *curve,
+                                                                 float                   progress,
+                                                                 graphene_vec2_t        *normal);
 void                    gsk_curve_split                         (const GskCurve         *curve,
                                                                  float                   progress,
                                                                  GskCurve               *start,