shell: Remove the black curtain installed on outputs

shell: Remove the black curtain installed on outputs

Upon completion of activation we might have an output without
a background surface set-up on that particular output, so this patch
either uses the black curtain as a background surface (useful for
transparent applications, which need it) or we remove it entirely in case
the shell client has one installed.

The patch renames the black_surface to black_curtain to better reflect
its name. Also output filtering would need renaming
(desktop_surface_check_last_surfaces)

Bug-AGL: SPEC-4516
Reported-by: Vasyl Vavrychuk <vasyl.vavrychuk@opensynergy.com>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I6071b07664772d891a4d8c33a7603573ec2069fa