Implement CHANGES style pixel placer packet

[Zekiah-A]

See #301

The pixel placers data is quite big uncompressed (since user ids are uint32s, so the data size is canvas width canvas height 4). So the current implementation allows the client to just request the pixel placer IDs around the current cursor position when it needs it (so basically when you hover over a pixel for a few ms it will start loading them in). I am also considering implementing a packet like CHANGES, where pixel placer IDs since the last canvas backup are pushed to the git repo, and then the server sends all the pixel placer IDs since the backup in a compressed buffer

TODO:

• Compare bandwidth of two solutions, and decide on if this should be implemented
• This might also require making intIdPositions a uint32 array, as using a map when the client is downloading all pixel placers in this situation would be beyond stupid

[Zekiah-A]

Current solution works well and is satisfactory. Closing