Checksum comparison

[HalfdanJ]

This PR is adding a checksum comparison of the image returned from the server, and only triggers display refresh if image has changed. This allows a higher refresh-rate, wither unnecessary flickering on the display.

[maxbbraun]

Can you say more about what motivated this change? If you're frequently waking up the client only to push the same image, then maybe the solution is to avoid that server-side? With this proposed change, you'd avoid the flickering, but you'd still incur the cost on battery life.