shell: introduce shell_surface_purpose

Enumerate the different surface purposes for the shell: background, panel,
lock surface, and normal (other) surfaces.

Instead of testing wlsc_surface pointers against known pointers, check
the purpose field.

This change will ease implementing per-output background, panel, etc.
when the number of "known" surface pointers grows dynamically.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
1 file changed