Port increment_move radius clamps from Ring Racers

Fixes inconsistencies with thin players, like with Liftoff Gantry's scale section, and with large characters colliding with steep slopes instead of going up them.

src/p_map.c

@@ -2728,8 +2728,22 @@ increment_move
 	fixed_t thingtop;
 	floatok = false;
 
-	if (radius < MAXRADIUS/2)
-		radius = MAXRADIUS/2;
+	// This makes sure that there are no freezes from computing extremely small movements.
+	// Originally was MAXRADIUS/2, but that can cause some bad inconsistencies for small players.
+	radius = max(radius, thing->scale);
+
+	// And we also have to prevent Big Large (tm) movements, as those can skip too far
+	// across slopes and cause us to fail step up checks on them when we otherwise shouldn't.
+	radius = min(radius, 16 * thing->scale);
+
+	// (This whole "step" system is flawed; it was OK before, but the addition of slopes has
+	// exposed the problems with doing it like this. The right thing to do would be to use
+	// raycasting for physics to fix colliding in weird order, double-checking collisions,
+	// randomly colliding with slopes instead of going up them, etc. I don't feel like porting
+	// that from RR, as its both a huge sweeping change and still incomplete at the time of
+	// writing. Clamping radius to make our steps more precise will work just fine as long
+	// as you keep all of your crazy intentions to poke any of the other deep-rooted movement
+	// code to yourself. -- Sal 6/5/2023)
 
 	do {
 		if (thing->flags & MF_NOCLIP) {