Handle meta packet and return ack. server scroller

The scroller isn't usable in production as the client will deselect the
server listing each time the list is updated. This prevents you from
selecting a server.

common/src/main/scala/net/psforever/packet/control/SlottedMetaAck.scala

@@ -0,0 +1,35 @@
+// Copyright (c) 2016 PSForever.net to present
+package net.psforever.packet.control
+
+import net.psforever.packet.{ControlPacketOpcode, Marshallable, PlanetSideControlPacket}
+import scodec.Codec
+import scodec.bits.{BitVector, ByteOrdering, ByteVector}
+import scodec.codecs._
+
+final case class SlottedMetaAck(slot : Int, subslot : Int)
+  extends PlanetSideControlPacket {
+  type Packet = SlottedMetaAck
+
+  assert(slot >= 0 && slot <= 7, s"Slot number ($slot) is out of range")
+
+  def opcode = {
+    val base = ControlPacketOpcode.RelatedB0.id
+    ControlPacketOpcode(base + slot % 4)
+  }
+
+  // XXX: a nasty hack to ignore the "slot" field
+  // There is so much wrong with this it's not even funny. Why scodec, whyyyy...
+  // I've never had a library make me feel so stupid and smart at the same time
+  def encode = SlottedMetaAck.encode(this).map(vect => vect.drop(8))
+}
+
+object SlottedMetaAck extends Marshallable[SlottedMetaAck] {
+  implicit val codec : Codec[SlottedMetaAck] = (
+    ("slot" | uint8L.xmap[Int](a => a - ControlPacketOpcode.RelatedB0.id, a=>a) ) ::
+      ("subslot" | uint16)
+    ).as[SlottedMetaAck]
+
+  def decodeWithOpcode(slot : ControlPacketOpcode.Value)(bits : BitVector) = {
+    decode(ControlPacketOpcode.codec.encode(slot).require ++ bits)
+  }
+}