Fix T54686: objects don't occlude each other for edit-mesh select (part 2)

The previous fix 8a6414ed46f66, resolved selection picking but didn't
work for box/circle/lasso select.

- Add ED_select_buffer_utils.h for general select-buffer operations
  unrelated to edit-mesh.

- Circle select still needs to cache select-id's for each update.

1 files changed, 42 insertions, 3 deletions

diff --git a/source/blender/editors/mesh/editmesh_select.c b/source/blender/editors/mesh/editmesh_select.c
index 372ff42f1c3..d6a32a6bd70 100644
--- a/source/blender/editors/mesh/editmesh_select.c
+++ b/source/blender/editors/mesh/editmesh_select.c
@@ -199,8 +199,19 @@ void EDBM_automerge(Scene *scene, Object *obedit, bool update, const char hflag)
  * \{ */
 
 struct EDBMBaseOffset {
-  uint face;
-  uint edge;
+  /* For convenience only. */
+  union {
+    uint offset;
+    uint face_start;
+  };
+  union {
+    uint face;
+    uint edge_start;
+  };
+  union {
+    uint edge;
+    uint vert_start;
+  };
   uint vert;
 };
 
@@ -209,6 +220,8 @@ struct EDBMSelectID_Context {
   /** Borrow from caller (not freed). */
   struct Base **bases;
   uint bases_len;
+  /** Total number of items `base_array_index_offsets[bases_len - 1].vert`. */
+  uint base_array_index_len;
 };
 
 static bool check_ob_drawface_dot(short select_mode, const View3D *v3d, char dt)
@@ -234,7 +247,7 @@ static void edbm_select_pick_draw_bases(struct EDBMSelectID_Context *sel_id_ctx,
   Scene *scene_eval = (Scene *)DEG_get_evaluated_id(vc->depsgraph, &vc->scene->id);
   DRW_framebuffer_select_id_setup(vc->ar, true);
 
-  uint offset = 0;
+  uint offset = 1;
   for (uint base_index = 0; base_index < sel_id_ctx->bases_len; base_index++) {
     Object *ob_eval = DEG_get_evaluated_object(vc->depsgraph,
                                                sel_id_ctx->bases[base_index]->object);
@@ -252,9 +265,12 @@ static void edbm_select_pick_draw_bases(struct EDBMSelectID_Context *sel_id_ctx,
                               &base_ofs->edge,
                               &base_ofs->face);
 
+    base_ofs->offset = offset;
     offset = base_ofs->vert;
   }
 
+  sel_id_ctx->base_array_index_len = offset;
+
   DRW_framebuffer_select_id_release(vc->ar);
 }
 
@@ -306,6 +322,29 @@ BMElem *EDBM_select_id_bm_elem_get(struct EDBMSelectID_Context *sel_id_ctx,
   }
 }
 
+uint EDBM_select_id_context_offset_for_object_elem(const struct EDBMSelectID_Context *sel_id_ctx,
+                                                   int base_index,
+                                                   char htype)
+{
+  struct EDBMBaseOffset *base_ofs = &sel_id_ctx->base_array_index_offsets[base_index];
+  if (htype == BM_VERT) {
+    return base_ofs->vert_start - 1;
+  }
+  if (htype == BM_EDGE) {
+    return base_ofs->edge_start - 1;
+  }
+  if (htype == BM_FACE) {
+    return base_ofs->face_start - 1;
+  }
+  BLI_assert(0);
+  return 0;
+}
+
+uint EDBM_select_id_context_elem_len(const struct EDBMSelectID_Context *sel_id_ctx)
+{
+  return sel_id_ctx->base_array_index_len;
+}
+
 struct EDBMSelectID_Context *EDBM_select_id_context_create(ViewContext *vc,
                                                            Base **bases,
                                                            uint bases_len,