Implement adding velocity to player from Lua

The intended usecase is knockback, but there's potential for more.

src/network/clientopcodes.cpp

@@ -67,7 +67,7 @@ const ToClientCommandHandler toClientCommandTable[TOCLIENT_NUM_MSG_TYPES] =
 	null_command_handler,
 	{ "TOCLIENT_TIME_OF_DAY",              TOCLIENT_STATE_CONNECTED, &Client::handleCommand_TimeOfDay }, // 0x29
 	{ "TOCLIENT_CSM_RESTRICTION_FLAGS",    TOCLIENT_STATE_CONNECTED, &Client::handleCommand_CSMRestrictionFlags }, // 0x2A
-	null_command_handler,
+	{ "TOCLIENT_PLAYER_SPEED",             TOCLIENT_STATE_CONNECTED, &Client::handleCommand_PlayerSpeed }, // 0x2B
 	null_command_handler,
 	null_command_handler,
 	null_command_handler,

src/network/clientpackethandler.cpp

@@ -1383,6 +1383,17 @@ void Client::handleCommand_CSMRestrictionFlags(NetworkPacket *pkt)
 	loadMods();
 }
 
+void Client::handleCommand_PlayerSpeed(NetworkPacket *pkt)
+{
+	v3f added_vel;
+
+	*pkt >> added_vel;
+
+	LocalPlayer *player = m_env.getLocalPlayer();
+	assert(player != NULL);
+	player->addVelocity(added_vel);
+}
+
 /*
  * Mod channels
  */

src/network/networkprotocol.h

@@ -194,6 +194,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 		New network float format
 		ContentFeatures version 13
 		Add full Euler rotations instead of just yaw
+		Add TOCLIENT_PLAYER_SPEED
 */
 
 #define LATEST_PROTOCOL_VERSION 37
@@ -295,6 +296,11 @@ enum ToClientCommand
 		u32 CSMRestrictionFlags byteflag
 	 */
 
+	TOCLIENT_PLAYER_SPEED = 0x2B,
+	/*
+		v3f added_vel
+	 */
+
 	// (oops, there is some gap here)
 
 	TOCLIENT_CHAT_MESSAGE = 0x2F,

src/network/serveropcodes.cpp

@@ -156,7 +156,7 @@ const ClientCommandFactory clientCommandFactoryTable[TOCLIENT_NUM_MSG_TYPES] =
 	null_command_factory,
 	{ "TOCLIENT_TIME_OF_DAY",              0, true }, // 0x29
 	{ "TOCLIENT_CSM_RESTRICTION_FLAGS",    0, true }, // 0x2A
-	null_command_factory,
+	{ "TOCLIENT_PLAYER_SPEED",             0, true }, // 0x2B
 	null_command_factory,
 	null_command_factory,
 	null_command_factory,