src/cursor.c: fix invisible cursor on output loss / restore

src/cursor.c: fix invisible cursor on output loss / restore

Previously, the cursor image was only updated on output loss when the
cursor was on a labwc owned surface. This patch forces a re-enter of a
client surface in the remaining case of cursor being over a non-labwc
surface which causes the client to re-set its own cursor image.

This fixes a regression caused by 4dc99e2f3856650f3b61778f015a823cfae978d2.
Thanks to @Flrian for finding the root cause of the issue.

Fixes #820

Reported-by: @Flrian
Tested-by: @Flrian