This changes headless operation to strictly be a runtime option, rather
than a compile-time one. Note that the old headless version still relied
on a display server to support WebGL, while it now requires one all the
time.
Fixes#8573
Also change when pipelines become active.
This makes the constellation activate a pipeline as the current frame
when it is ready to do initial reflow, rather than when it is ready
to paint.
This fixes a number of intermittent failures that could previously occur
if an iframe was not visible - which would mean it was never moved from
a pending frame in the constellation to an active frame.
(It happens that webrender exposes these intermittents as permanent failures).
