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.
2 jobs for viewport-scrolling in 2 minutes and 25 seconds (queued for 41 seconds)
latest
Status Job ID Name Coverage
  Container Prep
passed #555010
container_prep

00:00:36

 
  Build
passed #555011
build-native-meson

00:01:49