libweston: Add new scene-graph

Adds a new scene-graph, scene-graph-by-output, which is the same as
scene-graph but groups views by outputs first then by layers. Arguably
it seems much more useful when multiple outputs are in use.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
3 jobs for scene-graph-reason-for-compositing in 1 minute and 23 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Container Prep
passed #1130861
container_prep

00:00:24

 
  Build
passed #1130862
build-native-meson-default-options

00:00:53

passed #1130863
build-native-meson-no-gl-renderer

00:00:33