Fix VVV_isxdigit() accepting A-Za-z instead of A-Fa-f

This function accepted more characters than it should have. (I made the
same mistake on my PR to SDL, embarassingly enough...)

desktop_version/src/UtilityClass.cpp

@@ -290,8 +290,8 @@ bool is_number(const char* str)
 
 static bool VVV_isxdigit(const unsigned char digit)
 {
-	return (digit >= 'a' && digit <= 'z')
-	|| (digit >= 'A' && digit <= 'Z')
+	return (digit >= 'a' && digit <= 'f')
+	|| (digit >= 'A' && digit <= 'F')
 	|| SDL_isdigit(digit);
 }