Refactor: DNA: Add util to access size of a truct.

Avoids reafile code having to access 'internals' specifics of DNA data.

source/blender/blenloader/intern/readfile.cc

@@ -1697,7 +1697,8 @@ static void switch_endian_structs(const SDNA *filesdna, BHead *bhead)
   char *data;
 
   data = (char *)(bhead + 1);
-  blocksize = filesdna->types_size[filesdna->structs[bhead->SDNAnr]->type];
+
+  blocksize = DNA_struct_size(filesdna, bhead->SDNAnr);
 
   nblocks = bhead->nr;
   while (nblocks--) {

source/blender/blenloader/intern/writefile.cc

@@ -738,9 +738,7 @@ static void writestruct_at_address_nr(
   bh.nr = nr;
 
   bh.SDNAnr = struct_nr;
-  const SDNA_Struct *struct_info = wd->sdna->structs[bh.SDNAnr];
-
-  bh.len = nr * wd->sdna->types_size[struct_info->type];
+  bh.len = nr * DNA_struct_size(wd->sdna, bh.SDNAnr);
 
   if (bh.len == 0) {
     return;

source/blender/makesdna/DNA_genfile.h

@@ -192,6 +192,13 @@ int DNA_struct_member_size(const struct SDNA *sdna, short type, short name);
  */
 int DNA_elem_type_size(eSDNA_Type elem_nr);
 
+/**
+ * Returns the size of a struct.
+ *
+ * \param struct_index: Index into the #sdna.structs array (aka #BHead.SDNAnr).
+ */
+int DNA_struct_size(const struct SDNA *sdna, int struct_index);
+
 /**
  * Get the alignment that should be used when allocating memory for this type.
  */

source/blender/makesdna/intern/dna_genfile.cc

@@ -156,6 +156,11 @@ void DNA_sdna_free(SDNA *sdna)
   MEM_freeN(sdna);
 }
 
+int DNA_struct_size(const SDNA *sdna, int struct_index)
+{
+  return sdna->types_size[sdna->structs[struct_index]->type];
+}
+
 /**
  * Return true if the name indicates a pointer of some kind.
  */