comment noise2d bitshift

While working through the code I was momentarily concerned that the right bitshift in noise2d could fill ones in some cases. It turns out that with signed integers, this is indeed true, but this one is shifting an unsigned integer, so the behavior is as expected. I put a comment here to clarify this, in case someone else wonders the same thing down the line.

src/noise.cpp

@@ -167,6 +167,7 @@ float noise2d(int x, int y, s32 seed)
 {
 	unsigned int n = (NOISE_MAGIC_X * x + NOISE_MAGIC_Y * y
 			+ NOISE_MAGIC_SEED * seed) & 0x7fffffff;
+	// Right bit shift on an unsigned type zero-fills.
 	n = (n >> 13) ^ n;
 	n = (n * (n * n * 60493 + 19990303) + 1376312589) & 0x7fffffff;
 	return 1.f - (float)(int)n / 0x40000000;