prevent SSinput from constantly reporting null movements (#59558) (#6208)

Just prevents SSinput from constantly calling Move() every tick for every user even when no actual movement is requested.

Only done rudimentary testing - straightforward and we've been using this for months on CM13 but without diagonal movement.

I asked the original SSinput author back then which said that it was likely an oversight. I believe moving the diagonal movement direction buffer reset to keyLoop instead of Move() also makes sense as it is actually relevant to SSinput and not client actions as a whole (eg. if you call built-in movement verbs)

Co-authored-by: fira <loyauflorian@gmail.com>

code/modules/keybindings/bindings_atom.dm

@@ -7,8 +7,13 @@
 		movement_dir = movement_dir | user.movement_keys[_key]
 	if(user.next_move_dir_add)
 		movement_dir |= user.next_move_dir_add
+		user.next_move_dir_add = 0
 	if(user.next_move_dir_sub)
 		movement_dir &= ~user.next_move_dir_sub
+		user.next_move_dir_sub = 0
+	if(!movement_dir)
+		return
+
 	// Sanity checks in case you hold left and right and up to make sure you only go up
 	if((movement_dir & NORTH) && (movement_dir & SOUTH))
 		movement_dir &= ~(NORTH|SOUTH)

code/modules/mob/mob_movement.dm

@@ -69,9 +69,6 @@
 /client/Move(new_loc, direct)
 	if(world.time < move_delay) //do not move anything ahead of this check please
 		return FALSE
-	else
-		next_move_dir_add = 0
-		next_move_dir_sub = 0
 	var/old_move_delay = move_delay
 	move_delay = world.time + world.tick_lag //this is here because Move() can now be called mutiple times per tick
 	if(!mob || !mob.loc)