Implement the basics of screensaver surface management. Exec'ing and
killing the screensaver client is punted for later.
When a surface registered as a screensaver is mapped, it stays hidden
if the screen is not locked, or it is added to the compositor visible
surfaces list if the screen is locked.
The map() is restructured to set initial position first, and stacking
next. This allows SHELL_SURFACE_SCREENSAVER share positioning with
SHELL_SURFACE_FULLSCREEN, while show_screensaver() does its own
wlsc_surface_configure() call.
Also fix centering to the given fullscreen output, not the first output.
Another bug fix: previously configure() would call
wlsc_surface_configure() unconditionally, which assigns an output to
the surface. While the compositor is locked, if an application resizes
its window, we hit configure() and assign an output while the surface is
not in compositor->surface_list. This leads to invalid memory access on
the next call to wlsc_surface_damage_below(). Fix this by not calling
wlsc_surface_configure() for surfaces that are not visible.
Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
77346a66ec