Add gsk_path_get_previous/next_point

These functions let you iterate over the 'significant'
points of a path. What makes these functions nice is that
you don't need a measure to obtain these points.

gsk/gskpath.c

diff --git a/gsk/gskpath.c b/gsk/gskpath.c
index 1804c83..dc0b2e3 100644
--- a/gsk/gskpath.c
+++ b/gsk/gskpath.c
@@ -551,6 +551,122 @@
 }
 
 /**
+ * gsk_path_get_previous_point:
+ * @self: a `GskPath`
+ * @point: a point on @self
+ * @result: (out caller-allocates): Return location for the result
+ *
+ * Gets the previous 'significant' point on @self before @point.
+ *
+ * The 'significant' points of a path are typically the
+ * on-curve points that have been specified when the
+ * path was created.
+ *
+ * For example, in a path with 3 Bézier segments, the
+ * significant points are the start of the first segment,
+ * the start point of the second segment (which coincides
+ * with the end point of the first segment), the start
+ * point of the third segment, and the end point of the
+ * last segment.
+ *
+ * If @point is the start point of the path, there is no
+ * prior point, and this function returns `FALSE`.
+ *
+ * Returns: `TRUE` if @result has been set to a point
+ *
+ * Since: 4.20
+ */
+gboolean
+gsk_path_get_previous_point (GskPath            *self,
+                             const GskPathPoint *point,
+                             GskPathPoint       *result)
+{
+  g_return_val_if_fail (self != NULL, FALSE);
+
+  if (point->t > 0)
+    {
+      result->contour = point->contour;
+      result->idx = point->idx;
+      result->t = 0;
+      return TRUE;
+    }
+  else if (point->idx > 0)
+    {
+      result->contour = point->contour;
+      result->idx = point->idx - 1;
+      result->t = 0;
+      return TRUE;
+    }
+  else if (point->contour > 0)
+    {
+      result->contour = point->contour - 1;
+      result->idx = gsk_contour_get_n_ops (self->contours[result->contour]) - 1;
+      result->t = 1;
+      return TRUE;
+    }
+
+  return FALSE;
+}
+
+/**
+ * gsk_path_get_next_point:
+ * @self: a `GskPath`
+ * @point: a point on @self
+ * @result: (out caller-allocates): Return location for the result
+ *
+ * Gets the next 'significant' point on @self after @point.
+ *
+ * The 'significant' points of a path are typically the
+ * on-curve points that have been specified when the
+ * path was created.
+ *
+ * For example, in a path with 3 Bézier segments, the
+ * significant points are the start of the first segment,
+ * the start point of the second segment (which coincides
+ * with the end point of the first segment), the start
+ * point of the third segment, and the end point of the
+ * last segment.
+ *
+ * If @point is the end point of the path, there is no
+ * next point, and this function returns `FALSE`.
+ *
+ * Returns: `TRUE` if @result has been set to a point
+ *
+ * Since: 4.20
+ */
+gboolean
+gsk_path_get_next_point (GskPath            *self,
+                         const GskPathPoint *point,
+                         GskPathPoint       *result)
+{
+  g_return_val_if_fail (self != NULL, FALSE);
+
+  if (point->t < 1)
+    {
+      result->contour = point->contour;
+      result->idx = point->idx;
+      result->t = 1;
+      return TRUE;
+    }
+  else if (point->idx < gsk_contour_get_n_ops (self->contours[point->contour]) - 1)
+    {
+      result->contour = point->contour;
+      result->idx = point->idx + 1;
+      result->t = 1;
+      return TRUE;
+    }
+  else if (point->contour < self->n_contours - 1)
+    {
+      result->contour = point->contour + 1;
+      result->idx = 0;
+      result->t = 0;
+      return TRUE;
+    }
+
+  return FALSE;
+}
+
+/**
  * gsk_path_get_closest_point:
  * @self: a path
  * @point: the point

gsk/gskpath.h

diff --git a/gsk/gskpath.h b/gsk/gskpath.h
index ce220b3..78c2f76 100644
--- a/gsk/gskpath.h
+++ b/gsk/gskpath.h
@@ -143,6 +143,16 @@
                                                                  GskPathForeachFunc      func,
                                                                  gpointer                user_data);
 
+GDK_AVAILABLE_IN_4_20
+gboolean                gsk_path_get_previous_point             (GskPath                *self,
+                                                                 const GskPathPoint     *point,
+                                                                 GskPathPoint           *result);
+
+GDK_AVAILABLE_IN_4_20
+gboolean                gsk_path_get_next_point                 (GskPath                *self,
+                                                                 const GskPathPoint     *point,
+                                                                 GskPathPoint           *result);
+
 /**
  * GskPathIntersection:
  * @GSK_PATH_INTERSECTION_NONE: No intersection