Fix #6958: Recent messages are not coloured

This fix is a workaround to what I believe is a bug in MSVC's optimised code generator. Somehow the casting of a wchar_t to a sint32 is causing some strange behaviour where the number 147 gets changed to 0x??93. Adding a printf to display codepoint was enough to alter the behaviour, making it very difficult to understand what was going on.

src/openrct2/localisation/Convert.cpp

@@ -20,7 +20,7 @@
 
 sint32 rct2_to_utf8(utf8 *dst, const char *src)
 {
-    sint32 codepoint;
+    wchar_t codepoint;
 
     utf8 *start = dst;
     const char *ch = src;