Change Colour constructors to be constexpr

src/gfx_type.h

@@ -180,7 +180,7 @@ union Colour {
 	 * @param b The channel for the blue colour.
 	 * @param a The channel for the alpha/transparency.
 	 */
-	Colour(uint8 r, uint8 g, uint8 b, uint8 a = 0xFF) :
+	constexpr Colour(uint8 r, uint8 g, uint8 b, uint8 a = 0xFF) :
 #if defined(__EMSCRIPTEN__)
 		r(r), g(g), b(b), a(a)
 #elif TTD_ENDIAN == TTD_BIG_ENDIAN
@@ -195,7 +195,7 @@ union Colour {
 	 * Create a new colour.
 	 * @param data The colour in the correct packed format.
 	 */
-	Colour(uint data = 0) : data(data)
+	constexpr Colour(uint data = 0) : data(data)
 	{
 	}
 };