Fix openMP build

Tested to compile on VC12 with openMP activated.

With two loops over the same unit list, 'guided' scheduling (as done
before the second loop was introduced) isn't optimal.

[ci skip]

src/display.cpp

@@ -3098,13 +3098,12 @@ void display::invalidate_animations()
 		open_mp_list.push_back(u);
 	}
 
-	#pragma omp parallel shared(chunks)
-	{
-		#pragma omp for
-		for(size_t i = 0; i < open_mp_list.size(); ++i)
-			open_mp_list[i]->anim_comp().refresh();
+	// openMP can't iterate over size_t
+	const int omp_iterations = open_mp_list.size();
+	#pragma omp parallel for shared(open_mp_list)
+	for (int i = 0; i < omp_iterations; i++) {
+		open_mp_list[i]->anim_comp().refresh();
 	}
-}
 #endif
 
 
@@ -3119,14 +3118,12 @@ void display::invalidate_animations()
 			new_inval |=  u->anim_comp().invalidate(*this);
 		}
 #else
-		#pragma omp parallel shared(chunks)
-		{
-			#pragma omp for
-			for(size_t i = 0; i < open_mp_list.size(); ++i)
+	#pragma omp parallel for reduction(|:new_inval) shared(open_mp_list)
+		for (int i = 0; i < omp_iterations; i++) {
 				new_inval |= open_mp_list[i]->anim_comp().invalidate(*this);
 		}
 #endif
-	}while(new_inval);
+	} while (new_inval);
 }
 
 void display::add_arrow(arrow& arrow)