Allow skipping of terrain graphics rotations with a magic value "skip"

changelog

@@ -150,6 +150,8 @@ Version 1.13.5+dev:
    * [set_variable] now supports abs= (absolute value), power= (raise to
      power) and root= (extracts root, 'square' is the only value currently
      supported) keys
+   * Individual [terrain_graphics] rotations can now be skipped entirely by
+     using "skip" in the rotations= list.
  * Lua API:
    * Upgrade to Lua 5.3.3+
      Consult http://www.lua.org/ for a full change list and updated documenation.

src/terrain/builder.cpp

@@ -869,6 +869,11 @@ void terrain_builder::add_rotated_rules(building_ruleset &rules, building_rule &
 			   and remove it if invalid. But since the ratio is so
 			   low, the speedup is not worth the extra multiset
 			   manipulations. */
+
+			if (rot.at(angle) == "skip") {
+				continue;
+			}
+
 			building_rule rule = tpl;
 			rotate_rule(rule, angle, rot);
 			add_rule(rules, rule);