Screencast utility functions and examples

[pyneda]

Development guide

Link

Test on local before making the PR

go run ./lib/utils/simple-check

Overview

This PR introduces some higher level functions to help start a page screencast and receive the frames through a channel. It then leaves the final implementation of what to do with the frames to the user.

Main changes:

• New page functions (p *Page) StartScreencast(opts *ScreencastOptions) (chan []byte, error) and (p *Page) StopScreencast() error
• Added MJPEG screencast support in the ServeMonitor function
• WriteMJPEGFrame utility function
• Added an example of how to use the functionality and broadcast the frames using websocket and MJPEG
• Added an example of how to create a video file using FFmpeg