[Uptime] Re-assemble chunked synthetics images

[andrewvc]

In elastic/synthetics#285 we describe the method for saving space on duplicated screenshot images. This will require compositing fragmented images via canvas in the UI, as well as changing the way we query for screenshots.

After the spike in the link synthetics repo is complete we will need to address the kibana side. We also need to determine if it is more efficient to do this on the server side at the existing screenshot endpoints than in the browser with canvas.

[elasticmachine]

Pinging @elastic/uptime (Team:uptime)

[justinkambic]

This was resolved by #101122.

We also need to determine if it is more efficient to do this on the server side at the existing screenshot endpoints than in the browser with canvas.

We determined that performing this compositing with the Canvas API was the way to go. The native lib used in Synthetics would require the Kibana distribution to markedly increase in size, and put significant processing load on the Kibana server. Performing the computations on the client alleviate that pressure, and opens the door for further optimization to limit network latency and utilization.

[justinkambic]

We can skip this for the test plan, because it's a duplicate of #101122.