Damages and captures both trigger repaints of outputs. Some
configurations don't care about damages and use headless only for
captures. This commit adds a new feature to libweston that lets
outputs repaint only on captures, not on damages. The headless backend
enables that new feature when given a special refresh rate of 0 mHz.
Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>