Cleanup: Select Engine: Draw actual indices instead of 'Original Index'

The selection engine renders a uint texture with the selectable mesh indices.

When these meshes have modifiers, the original index is used for the texture.

However this does not seem to be necessary. The real mesh indexes can be used in the texture and, only at the end, the original index can be retrieved on the CPU itself.

Advantages of this approach:
- Optimizes the code to generate selection buffers.
- Makes the select id texture more informative as it identifies the real elements instead of the original ones.

@@ -17,6 +17,7 @@ enum MeshForeachFlag {
 
 void BKE_mesh_foreach_mapped_vert(
     const Mesh *mesh,
+    const bool use_orig_index,

@@ -94,3 +94,3 @@
       DRW_shgroup_call_no_cull(face_shgrp, geom_facedots, ob);
     }
-    *r_face_offset = initial_offset + em->bm->totface;
+    *r_face_offset = initial_offset + mesh->faces_num;

@@ -70,2 +70,3 @@
 
-bool DRW_select_buffer_elem_get(uint sel_id, uint *r_elem, uint *r_base_index, char *r_elem_type);
+bool DRW_select_buffer_elem_get(
+    uint sel_id, uint *r_elem, uint *r_base_index, char *r_elem_type, const bool use_orig_index);

@@ -435,2 +438,4 @@
   }
 
+  if (use_orig_index) {
+    Object *object = select_ctx->objects[base_index];

@@ -436,1 +439,4 @@
 
+  if (use_orig_index) {
+    Object *object = select_ctx->objects[base_index];
+    Mesh *mesh_eval = BKE_object_get_evaluated_mesh(object);

@@ -437,0 +440,4 @@
+  if (use_orig_index) {
+    Object *object = select_ctx->objects[base_index];
+    Mesh *mesh_eval = BKE_object_get_evaluated_mesh(object);
+    const int *orig_index = reinterpret_cast<const int *>(

@@ -437,0 +441,4 @@
+    Object *object = select_ctx->objects[base_index];
+    Mesh *mesh_eval = BKE_object_get_evaluated_mesh(object);
+    const int *orig_index = reinterpret_cast<const int *>(
+        CustomData_get_layer(elem_type == SCE_SELECT_FACE ? &mesh_eval->face_data :

@@ -396,0 +401,4 @@
+    case SCE_SELECT_EDGE:
+      return &mesh_eval->edge_data;
+    case SCE_SELECT_VERTEX:
+    default: