Complete changes from commit 0d56c45a in NXpicturestr.h (i.e. add a missing comment).  (Fixes ArcticaProject/nx-libs#38).

commit 0d56c45a was:
nx-X11: handle source pictures (those without a Drawable surface) gracefully.

Attributes GH PR #66: https://github.com/ArcticaProject/nx-libs/pull/66

This fixes the problem Mike Gabriel describes like this:

- Launch a session on a system with a single monitor. Enable Xinerama
  for this session.
- Open a desktop session in that session window (e.g. MATE or XFCE).
- Move the NX/MATE-or-XFCE session window around on that one
  monitor. Bump at the borders, so that the session window moves into
  the invisible parts around your monitor.

What you see is that the MATE-or-XFCE window manager will become
really busy with resizing the windows and panels in the NX session,
because moving the window over the physical borders of the display
will trigger resize events.

This is non-intuitive, I think.

Same with multi-monitors on the outside edges of the physical Xorg
RandR setup.

X2Go Agent: Add x2goagent.features file that informs X2Go clients about the new RandR based Xinerama feature.

 By design, when resuming a session, Xinerama can only be re-enabled
 by NX option parsing.

 Thus, this change introduces a "xinerama" NX option that can
 be loaded via an options file into NX agent when resuming
 a session. The new xinerame NX session option also allows
 switching on Xinerama at session startup via an option file.

 When implementing the new NX Xinerama support into clients (like
 TheQVD, X2Go Client, etc., this new xinerama NX option should be
 used for activating Xinerama in the NX / QVD / X2Go session.

Initialize XRandR based Xinerama extension properly when session is started with "-geometry fullscreen".

 No more xinerama faking, just use existing xrandr extension and initalize
 it properly. Xinerama then works automatically.

 Fixes ArcticaProject/nx-libs#23

library clean-up: Don't build libNX_Xinerama anymore. Use system's libXinerama shared library. (Fixes ArcticaProject/nx-libs#49).

 This commit goes along with a patch from Ulrich Sibiller who managed to
 move the Xinerama awareness for NX sessions into the Xserver code. This
 makes Xinerama support for NX in libNX_Xinerama.so obsolete.

 Fixes ArcticaProject/nx-libs#49

library clean-up: Don't build libNX_Xtst anymore. Use system's libXtst shared library. (Fixes ArcticaProject/nx-libs#32).

Fix for too overall Xdamage and Xrandr client lib removal: Bring back the nx-x11proto-{randr,damage} header files and use those at compile time.

X11/nx-X11 clear-up: More places where we need to explicitly distinguish between X11 and nx-X11 namespace regarding header include paths.

pointer typedef in Xdefs.h: also catch Xdefs.h include in include/misc.h, make undoing the FIXME more obvious.

libray clean (Xfixes): Provide a fake libXfixes.h header from latest X.Org, that includes headers from our Xlib version. This avoids FTBFS caused by type redefinitions and should really be a temporary measure.

nxcompshad/X11/include/Xdamage_nxcompshad.h: Update FIXME statement. Work-around needs to persist until libX11 from X.Org gets used by nxagent.

pointer typedef in Xdefs.h: Define pointer type for building against older Xlib versions that still expect the pointer type to exist (e.g. Ubuntu <= 14.04, Debian <= 7.0).

Attributes GH PR #61: https://github.com/ArcticaProject/nx-libs/pull/61

 In the process of building nxagent against more and more system-wide installed
 X.org libraries, we come to the limit of including structs from this (bundled
 nx-X11) and that (system-wide X.Org) library.

 This commit introduces a clear namespace separation of headers provided by
 nx-X11 and headers provided by X.Org. This approach is only temporary as we
 want to drop all nx-X11 bundled libraries from nx-libs.

 However, for a while we need to make this separation clear and also ship
 some reduced fake X.Org headers that avoid pulling in libX* and libNX_X*
 symbols at the same time.

 This patch has been tested on Debian jessie and unstable and requires no
 overall testing on various distros and distro versions, as we finally will
 drop all libNX_X* libraries and build against X.org's client libs.

 For now, this hack eases our development / cleanup process.

Attributes GH PR #58: https://github.com/ArcticaProject/nx-libs/pull/58

Flagged by cppcheck 1.62:
[dix/dixfonts.c:1792]: (error) Common realloc mistake:
 'font_path_string' nulled but not freed upon failure
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Keith Packard <keithp@keithp.com>
Rebased against NX: Mike Gabriel <mike.gabriel@das-netzwerkteam.de>

Attributes GH PR #54: https://github.com/ArcticaProject/nx-libs/pull/54