shell: Don't map surfaces of type SHELL_SURFACE_NONE
shell: Don't map surfaces of type SHELL_SURFACE_NONE

When commit 07926d90 factored out the code that chooses in which layer
a surface is added to, it changed the behavior for surfaces with no
type. Instead of not adding it to any layer, the surface is added to
the current workspace.

This patch restores the old behavior.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=77527

Tarnyko/weston-xdg_surface_present,eyolfson/weston,udoprog/weston,xorgy/weston,Fantu/compositor-spice,jonnylamb/weston,kwm81/weston,sir-murray/weston,giucam/weston,krezovic/weston,Gnurou/weston,sir-murray/weston,xorgy/weston,mchalupa/weston,krezovic/weston,udoprog/weston,Fantu/compositor-spice,Fantu/compositor-spice,jonnylamb/weston,giucam/weston,Tarnyko/weston-xdg_surface_present,mchalupa/weston,eyolfson/weston,kwm81/weston,Gnurou/weston