libweston/backends: Move damage flush into backends
Currently we flush damage for the "primary plane" every repaint, but this is folly.
The drm backend may skip rendering entirely if using an all-planes composition. This could leave the renderer plane in a messy state if a surface on an overlay plane disappears.
Instead, let the backends flush the primary plane damage when they know they need to render.
Fixes #864 (closed)
Signed-off-by: Derek Foreman derek.foreman@collabora.com