nx-X11/programs/Xserver/hw/nxagent/Render.c: use macros obtaining values from _XPrivDisplay instead of the other Display structure directly.

Fixes: ArcticaProject/nx-libs#512

It's unclear to me why (Display*)(dpy)->bitmap_pad and
(_XPrivDisplay)(dpy)->bitmap_pad produce different results.

Technically, Display is referencing the _XDisplay/Display structure
defined in Xlibint.h, while _XPrivDisplay is a typedef'd pointer to the
(anonymous) structure defined in Xlib.h, since we're not defining the
XLIB_ILLEGAL_ACCESS macro. The weird part is that both should be equal
when it comes to its bitmap_pad, byte_order and bitmap_bit_order
members.