Xnest variant of ModifyPixmapHeader that creates new Pixmap in parent X
server if it's size is modified from 0x0 to anything bigger.
xnestCreatePixmap doesn't create pixmap in parent X server if it has
dimensions 0x0. If it is later resized and accessed, Xnest will be
aborted with BadDrawable error from parent X server because it will
use XID 0. This happens with ScratchPixmap, for example as used from
XaceCensorImage. Applications using XACE crash Xnest.
Reviewed-by: Adam Jackson <ajax@redhat.com>
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