Add POS_MOD macro and use for all positive modulos

This macro is to make it so it won't be error-prone to write the
semi-confusing `(a % b + b) % b` statement, and you can just use an easy
macro instead.

Currently, the only places a positive modulo is needed is when switching
tilesets, enemies, and warp directions in the editor, as well as when
getting a tile in the tower, since towers just repeat themselves
vertically. Towers used this weird while-loop to sort of emulate a
modulo, which isn't half-bad, but is unnecessary, and I don't think any
compiler would recognize it as a modulo. (And if it's not optimized to a
proper modulo... what happens if the number being moduloed is really,
really big?)

desktop_version/src/UtilityClass.h

@@ -88,6 +88,11 @@ void _VVV_between(
     } \
     while (false)
 
+/* Positive modulo, because C/C++'s modulo operator sucks and is negative given
+ * negative divisors.
+ * WARNING! This double- and triple- evaluates. */
+#define POS_MOD(a, b) (((a) % (b) + (b)) % (b))
+
 //helperClass
 class UtilityClass
 {