bots only get one full update when joining to save bandwidth and
computation time in json and gzip.

redraw snake in colour derived from player.id, not current colour.
current colour might be different if snake is invincible.

invicible: only reset colour if snake is not invincible anymore,
instead of whenever 1 'stack' of invincibility ends.

failing validation is the only cause of disconnection before RegisterPlayer.
In this case we can not use the generic deregisterPlayer since not all parts
of the player are yet allocated.

can not tunnel through solidwall. must return a collision
in this case, otherwise move would attempt an out of bounds movement

bots do not lose their state during a dialog

this was a bug introduced when moving the player.reader onto
inputKeys, ProcessInput was disabled during dialogs by other means
before

player.reader provided io.Reader access to readBytes. A
dialog which used the reader, first stopped inputLoop, to
stop it from taking player.readers input. Then the dialog would
read from the reader. This was racy, because inputloop would read all
input after a 'start dialog' key, before the loop was stopped,
even though this input was meant for the dialog.

the reader now only rivals with ProcessInput, which can be synchronously
disabled.

always select disconnect first

previously rendertargets could react to disconnects in their
disconnect function, which was called asynchronously. however
the bot's gzip writer does not support concurrent access, so
we moved this functionality into a synchronous deregister function.

when newplayer is called, the players renderTarget
is still nil. readloop can call disconnect asynchronously,
which requires player.renderTarget to be set

instead of using one data interface{} and determine it's
type by a type switch, add one struct member for every
possible type, this make parsing on the client side simpler,
since unmarshalling does not have to be nested anymore

bot plugins are not anymore processed by RegisterPlugin but
by RegisterPlayer. RegisterPlayer now calls a synchronous
initialisation function on the RenderTarget

allows clients to play when the board fits into their terminal but
the playerlist does not.

use UpdatePlayerListEntry and add clear functionality to UpdatePlayerListEntry

if no position is save, give out any empty position.
if no position is empty either, return an error and do not spawn
a snake