Fix: BLI: Correct total_size for sliced offset indices

Its common to have 0 at the begin of an offset indices sequence.
But this is not true for a slice of offset indices.

Pull Request: https://projects.blender.org/blender/blender/pulls/126964

source/blender/blenlib/BLI_offset_indices.hh

@@ -48,7 +48,7 @@ template<typename T> class OffsetIndices {
   /** Return the total number of elements in the referenced arrays. */
   T total_size() const
   {
-    return offsets_.size() > 1 ? offsets_.last() : 0;
+    return offsets_.size() > 1 ? offsets_.last() - offsets_.first() : 0;
   }
 
   /**