Footsteps: Fix offset footstep and shallow water sound bugs

Fix footstep sounds coming from nodes to either side when walking on a
1 node wide row of nodebox slabs such as default:snow.
Fix sand footsteps when swimming in 1 node deep water.

Use a new function 'getFootstepNodePos()' instead of 'getStandingNodePos()'
to avoid using a horizontally-offset 'sneak node' for sounds.

Sound is selected from the node BS * 0.05 below the player's feet, so
that 1/16th slabs will play the slab sound but 1/32nd slabs will not.
If the player is not 'touching ground' the node detection position is
changed to BS * 0.5 below to preserve footstep sounds when landing after
a jump or fall.

src/localplayer.h

@@ -68,6 +68,7 @@ public:
 	void applyControl(float dtime);
 
 	v3s16 getStandingNodePos();
+	v3s16 getFootstepNodePos();
 
 	// Used to check if anything changed and prevent sending packets if not
 	v3f last_position;