Added proper mod function.

src/engine/util/math.h

@@ -0,0 +1,8 @@
+// Copyright (c) 2021 Dominic Masters
+// 
+// This software is released under the MIT License.
+// https://opensource.org/licenses/MIT
+
+#pragma once
+
+#define mod(a,b) (a%b+b)%b