Add real-time video streaming support to Guacamole protocol

The existing video instruction is not sufficient for playback of real-time video, and by its own nature would induce latency, as it requires each video stream to be of known duration.

This instruction must be modified such that:

1. Blobs of video data can be received in real-time, without knowing the duration of the video stream beforehand (which may be indefinite).
2. Playback of video data occurs in real-time within the Guacamole client, through some callback function which is provided the associated input stream and sufficient access to the destination layer that arbitrary DOM elements can be appended and maintained (such as a <video> tag for sake of using Media Source Extensions).
3. Playback of video data can be synchronized with the rest of the Guacamole instruction stream based on received sync instructions, similar to how audio playback is synchronized using sync as of GUAC-427.

If it will be extremely common in codec implementations to create a <video> tag within a layer, this should somehow be automated.