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/game.cpp

@@ -3530,7 +3530,7 @@ void Game::updateSound(f32 dtime)
 	LocalPlayer *player = client->getEnv().getLocalPlayer();
 
 	ClientMap &map = client->getEnv().getClientMap();
-	MapNode n = map.getNodeNoEx(player->getStandingNodePos());
+	MapNode n = map.getNodeNoEx(player->getFootstepNodePos());
 	soundmaker->m_player_step_sound = nodedef_manager->get(n).sound_footstep;
 }