Fix Pose Brush origin position with large brush size

When the brush size is bigger than the entire mesh, fdata.tot_co can be
0, so the pose origin will default to (0,0,0), which does not make much
sense. After this patch, the pose origin will be set to the farthest
vertex from the pose origin, which at least should be in the surface of
the mesh and in most cases in the direction the pose brush was already
detecting the origin.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D7773

source/blender/editors/sculpt_paint/sculpt_pose.c

@@ -367,6 +367,10 @@ typedef struct PoseFloodFillData {
 
   bool is_first_iteration;
 
+  /* In topology mode this stores the furthest point from the stroke origin for cases when a pose
+   * origin based on the brush radius can't be set. */
+  float fallback_floodfill_origin[3];
+
   /* Fallback origin. If we can't find any face set to continue, use the position of all vertices
    * that have the current face set. */
   float fallback_origin[3];
@@ -377,12 +381,17 @@ static bool pose_topology_floodfill_cb(
     SculptSession *ss, int UNUSED(from_v), int to_v, bool is_duplicate, void *userdata)
 {
   PoseFloodFillData *data = userdata;
+  const float *co = SCULPT_vertex_co_get(ss, to_v);
 
   if (data->pose_factor) {
     data->pose_factor[to_v] = 1.0f;
   }
 
-  const float *co = SCULPT_vertex_co_get(ss, to_v);
+  if (len_squared_v3v3(data->pose_initial_co, data->fallback_floodfill_origin) <
+      len_squared_v3v3(data->pose_initial_co, co)) {
+    copy_v3_v3(data->fallback_floodfill_origin, co);
+  }
+
   if (sculpt_pose_brush_is_vertex_inside_brush_radius(
           co, data->pose_initial_co, data->radius, data->symm)) {
     return true;
@@ -521,12 +530,16 @@ void SCULPT_pose_calc_pose_data(Sculpt *sd,
   };
   zero_v3(fdata.pose_origin);
   copy_v3_v3(fdata.pose_initial_co, initial_location);
+  copy_v3_v3(fdata.fallback_floodfill_origin, initial_location);
   SCULPT_floodfill_execute(ss, &flood, pose_topology_floodfill_cb, &fdata);
   SCULPT_floodfill_free(&flood);
 
   if (fdata.tot_co > 0) {
     mul_v3_fl(fdata.pose_origin, 1.0f / (float)fdata.tot_co);
   }
+  else {
+    copy_v3_v3(fdata.pose_origin, fdata.fallback_floodfill_origin);
+  }
 
   /* Offset the pose origin. */
   float pose_d[3];