add Observable version of project()

[aplavin]

Feature description

The result of project(scene, point) depends on a lot of stuff, it's not enough to just do @lift project($scene, $point). One needs to do

@lift ($(scene.camera.projectionview); $(scene.viewport); project($scene, $point))

, and I'm not sure if that's the complete list. Is it specified anywhere?

Would be useful and less error-prone to add project_obs(scene, point)::Observable that takes care of including all proper dependencies of project().