Prepare rendering pipeline for viewer SubViews

• Streamlined internal pipeline interface to use RenderContext in all places
• Removed shadow cascade target from Shadow stage, moved to RenderScene
• Added shadow update mode setting to RenderScene
• Removed StageCamera and CameraBase as members of RenderPipeline

Changes in this MR are breaking if the old (non-RenderContext const& ctx) pipeline interface is still in use. Related MR for glow-samples: https://graphics.rwth-aachen.de:9000/Glow/glow-samples/merge_requests/3