1.7.10: Fix player position too high on login. Closes #87

Adds a new TeleportPlayer_NoGround packet, which is subtly different
from TeleportPlayer_NoConfirm. The flags u8 is replaced with an
on_ground bool, but more importantly the Y position is the eyes
position, so we have to translate to feet position for the client.

1.7.10: https://wiki.vg/index.php?title=Protocol&oldid=6003#Player_Position_And_Look
1.8.9: https://wiki.vg/index.php?title=Protocol&oldid=7368#Player_Position_And_Look

Author: iceiix

src/protocol/packet.rs

@@ -1310,6 +1310,14 @@ state_packets!(
                 field pitch: f32 =,
                 field flags: u8 =,
             }
+            packet TeleportPlayer_OnGround {
+                field x: f64 =,
+                field eyes_y: f64 =,
+                field z: f64 =,
+                field yaw: f32 =,
+                field pitch: f32 =,
+                field on_ground: bool =,
+            }
             /// EntityUsedBed is sent by the server when a player goes to bed.
             packet EntityUsedBed {
                 field entity_id: VarInt =,

src/protocol/versions/v1_7_10.rs

@@ -42,7 +42,7 @@ protocol_packet_ids!(
             0x05 => SpawnPosition_i32
             0x06 => UpdateHealth_u16
             0x07 => Respawn
-            0x08 => TeleportPlayer_NoConfirm
+            0x08 => TeleportPlayer_OnGround
             0x09 => SetCurrentHotbarSlot
             0x0a => EntityUsedBed_i32
             0x0b => Animation

src/server/mod.rs

@@ -415,6 +415,7 @@ impl Server {
                             MultiBlockChange_u16 => on_multi_block_change_u16,
                             TeleportPlayer_WithConfirm => on_teleport_player_withconfirm,
                             TeleportPlayer_NoConfirm => on_teleport_player_noconfirm,
+                            TeleportPlayer_OnGround => on_teleport_player_onground,
                             TimeUpdate => on_time_update,
                             ChangeGameState => on_game_state_change,
                             UpdateBlockEntity => on_block_entity_update,
@@ -1000,6 +1001,11 @@ impl Server {
         self.on_teleport_player(teleport.x, teleport.y, teleport.z, teleport.yaw as f64, teleport.pitch as f64, teleport.flags, None)
     }
 
+    fn on_teleport_player_onground(&mut self, teleport: packet::play::clientbound::TeleportPlayer_OnGround) {
+        let flags: u8 = 0; // always absolute
+        self.on_teleport_player(teleport.x, teleport.eyes_y - 1.62, teleport.z, teleport.yaw as f64, teleport.pitch as f64, flags, None)
+    }
+
     fn on_teleport_player(&mut self, x: f64, y: f64, z: f64, yaw: f64, pitch: f64, flags: u8, teleport_id: Option<protocol::VarInt>) {
         use std::f64::consts::PI;
         if let Some(player) = self.player {