UV: fix unreported, bad uv packing with convex shape_method.

Pivot wasn't set with convex shape_method when an island has only one triangle.

Regression from 86b1e5e3b6

source/blender/geometry/GEO_uv_pack.hh

@@ -88,6 +88,7 @@ class PackIsland {
   void finalize_geometry(const UVPackIsland_Params &params, MemArena *arena, Heap *heap);
 
  private:
+  void calculate_pivot(); /* Choose a pivot based on triangles. */
   blender::Vector<float2> triangle_vertices_;
   friend class Occupancy;
 };

source/blender/geometry/intern/uv_pack.cc

@@ -131,7 +131,8 @@ void PackIsland::finalize_geometry(const UVPackIsland_Params &params, MemArena *
   if (shape_method == ED_UVPACK_SHAPE_CONVEX) {
     /* Compute convex hull of existing triangles. */
     if (triangle_vertices_.size() <= 3) {
-      return; /* Trivial case, nothing to do. */
+      calculate_pivot();
+      return; /* Trivial case, calculate pivot only. */
     }
 
     int vert_count = int(triangle_vertices_.size());
@@ -156,7 +157,11 @@ void PackIsland::finalize_geometry(const UVPackIsland_Params &params, MemArena *
 
     BLI_heap_clear(heap, nullptr);
   }
+  calculate_pivot();
+}
 
+void PackIsland::calculate_pivot()
+{
   Bounds<float2> triangle_bounds = *bounds::min_max(triangle_vertices_.as_span());
   float2 aabb_min = triangle_bounds.min;
   float2 aabb_max = triangle_bounds.max;