Optimization: Client-side downscale in for pixel streaming

[eile]

Needs #18 to communicate window size on wall to client lib, which then scales down the image before transmission (e.g. in desktopstreamer). When the window is resized on the wall, a resize event will trigger retransmission from the client side.

[markolofsen]

@eile Have you tried the ready-made MetaEditor solution on React.js to demonstrate Unreal Engine in a browser with Pixel Streaming technology?