Fix: Ensure 31-bit shifts are unsigned. (#10128)

Shifting a signed 32-bit integer by 31 bits is undefined behaviour.
A few more than necessary are switched to unsigned for consistentency.

src/slope_type.h

@@ -81,7 +81,7 @@ DECLARE_ENUM_AS_BIT_SET(Slope)
  * Helper for creating a bitset of slopes.
  * @param x The slope to convert into a bitset.
  */
-#define M(x) (1 << (x))
+#define M(x) (1U << (x))
 /** Constant bitset with safe slopes for building a level crossing. */
 static const uint32 VALID_LEVEL_CROSSING_SLOPES = M(SLOPE_SEN) | M(SLOPE_ENW) | M(SLOPE_NWS) | M(SLOPE_NS) | M(SLOPE_WSE) | M(SLOPE_EW) | M(SLOPE_FLAT);
 #undef M