Cleanup: Remove global mesh threading size macro

The grain size depends on the algorithm, it doesn't make sense to use
the same value for multiple areas just because they relate to `Mesh`.
The comment mentions OpenMP which isn't used anymore.

Also remove an unused forward declaration.

@@ -80,0 +99,4 @@
+  float ref_beta;
+};
+
+/**

@@ -80,0 +102,4 @@
+/**
+ * Storage for coordinate spaces and connection information during normal calculation.
+ */
+struct NormalFanSpaces {

@@ -80,0 +109,4 @@
+   * size of each fan), there may be many fewer spaces than face corners, so they are stored in a
+   * separate array.
+   */
+  Array<NormalFanSpace> spaces;

@@ -80,0 +112,4 @@
+  Array<NormalFanSpace> spaces;
+
+  /** The index of the data in the #spaces array for each face corner. */
+  Array<int> corner_space_indices;

@@ -80,0 +115,4 @@
+  Array<int> corner_space_indices;
+};
+
+void fan_space_custom_normal_to_data(const NormalFanSpace *lnor_space,

@@ -572,2 +587,2 @@
-                                          const short clnor_data[2],
-                                          float r_custom_lnor[3])
+namespace blender::bke::mesh {
+static void fan_space_custom_data_to_normal(const NormalFanSpace *lnor_space,

@@ -82,0 +120,4 @@
+  Array<int> corner_space_indices;
+};
+
+void fan_space_custom_normal_to_data(const CornerNormalSpace *lnor_space,

@@ -1367,0 +1408,4 @@
+ * item. Group each item's indices together consecutively, encoding the grouping in #r_offsets,
+ * which is meant to be used by #OffsetIndices.
+ */
+static void reverse_index_array(const Span<int> item_indices,

@@ -1546,3 +1617,3 @@
   }
 
-  Vector<int> processed_corners;
+  Array<int> fan_corner_offset_indices;