e_comp_wl: clear pixmap image when client is unmapped and its object is hidden

e_comp_wl: clear pixmap image when client is unmapped and its object is hidden

 Uncommonly some clients's final buffer can be skipped if the client requests
unmap of its surface right after wl_surface@commit.
 So if evas_object of the client was hidden state and client was already
unmmapped, we can clear pixmap image mandatorily for preventing client
side rendering block(by absence of frame done event(wl_callback@done)).

Change-Id: Ibd2589f21dd46216ae2aef92a25cff6dde26c8b8
Signed-off-by: MinJeong Kim <minjjj.kim@samsung.com>