WIP clients: Implement viewport-scrolling demo

Implement a simple demo client that uses wp_viewport to scroll instead of repainting the buffer, based on simple-damage. The scrolling happens automatically for simplicity and the client also contains a software fallback for comparison.

Using the viewport on a once rendered buffer can hold substancial performance improvements/power savings, especially IO wise, over constant repainting by the client, as we can completely avoid buffer damage / texture blitting.