When resizing a window with redirected descendents, don't expose them

Bit/window gravity computations need to recompute exposures to manage the
bits which are saved by gravity during the resize computation. That's easy
for non-redirected windows where the bits are all within the parent's
pixmap. For redirected windows, we don't need to deal with this at all, so
just skip the whole re-computation adventure.

Signed-off-by: Keith Packard <keithp@keithp.com>