Move paint_all_new to backend.c

Fix build with -Dnew_backends=false. And more build test for this case.

Fixes #83

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/backend/backend.h

@@ -144,3 +144,4 @@ extern backend_info_t *backend_list[];
 
 bool default_is_win_transparent(void *, win *, void *);
 bool default_is_frame_transparent(void *, win *, void *);
+void paint_all_new(session_t *ps, region_t *region, win *const t);