workspace: Fix typo in META_IS_WORKSPACE macro
window: Fix crash when mapping sticky window

Since the introduction of set_workspace_state(), window->workspace
will always be NULL when on_all_workspaces is set - passing that
to a workspace function that does not validate its input will then
result in a crash.
Use the get_workspace() function instead, which will always return
a valid workspace.
window: Fix corner case in set_demands_attention()

We only grant requests to set the demands-attention hint if the window
is at least partially obscured; so for non-minimized windows on the
active workspace, we check if any other window on the same workspace
that is higher in the stack overlaps.
However in the case of a sticky window, window->workspace is NULL, so
we end up considering any non-sticky window on a different workspace.

At this point we have already established that the window is showing
on the active workspace, so use that to filter for windows that may
overlap.