118fab1 Don't drop length prefix for 1.7 encoding (#729)

Restore the VarInt packet length prefix after ViaRewind replaces Velocity's compression encoder with ForwardMessageToByteEncoder.

Without the length prefix, 1.7.x clients receive unframed packets and fail during login with "Bad packet id" errors.

Verified by reproducing issue #616 and confirming that adding the packet length framing allows a 1.7.10 client to proceed. Fixes https://github.com/ViaVersion/ViaRewind/issues/616