BLI: improve IndexRange

Defining `operator-` for the iterator allows it to be used in more generic
iterator code. The `index_range` method is the same that exists on many
other containers like `Vector`.

source/blender/blenlib/BLI_index_range.hh

@@ -105,6 +105,11 @@ class IndexRange {
       return a.current_ == b.current_;
     }
 
+    constexpr friend int64_t operator-(const Iterator &a, const Iterator &b)
+    {
+      return a.current_ - b.current_;
+    }
+
     constexpr int64_t operator*() const
     {
       return current_;
@@ -151,6 +156,11 @@ class IndexRange {
     return size_;
   }
 
+  constexpr IndexRange index_range() const
+  {
+    return IndexRange(size_);
+  }
+
   /**
    * Returns true if the size is zero.
    */