remote desktop: freeze while receiving large bitmap

[X512]

This is hrev54051.

When displaying large bitmap in remote desktop client (HTML5 or Haiku RemoteDesktop app) GUI freeze until bitmap receiving will be finished. Some kind of multiplexing of data streams of each rendering thread should be used.

[X512]

This is problem in current remote protocol, it currently multiplexed on message level, but messages can be large so other rendering threads will be locked until message receiving will finish.

Possible approaches:

1. Make all messages small. Bitmap message can be splitted to smaller messages such as 16x16 bitmaps.
2. Use stream based multiplexing. Each rendering thread will have stream id, and top level protocol format will be like int32 streamId; int32 payloadSize; int8 payload[payloadSize]. Concatenated payloads will contain messages for specific render thread.