so hide it behind that define

Attributes GH PR #842: https://github.com/ArcticaProject/nx-libs/pull/842

nxagentRenderTrap is rather old. This flag is set when internal pixmap
creation returns a pixmap with depth 0. Following that the RENDER
extension is effectively disabled. However: there's no code that could
create a pixmap with depth 0, so let's simplify our code by removing
nxagentRenderTrap altogether.

Fixes ArcticaProject/nx-libs#841

Attributes GH PR #844: https://github.com/ArcticaProject/nx-libs/pull/844

Attributes GH PR #835: https://github.com/ArcticaProject/nx-libs/pull/835

We should do something similar for all subsystems over time...

Especially switchin to/from fullscreen with active AutoGrab was
problematic. Works much smoother now.

This also make re-autograbbing after switch back from AllScreens work.

 You can now toggle between autograb mode by pressing CTRL-ALT-G
 (default, can be adjusted in keystrokes.cfg).

 Fixes ArcticaProject/nx-libs#384.

Attributes GH PR #834: https://github.com/ArcticaProject/nx-libs/pull/834

... not on reconnect. After the reconnect RRCloseScreen was called
twice which caused a double free. This was introduced with
3b06ad51

Fixes ArcticaProject/nx-libs#833

Attributes GH PR #826: https://github.com/ArcticaProject/nx-libs/pull/826

it is only used there, no need for a global variable

was only used once

It is only relevant there.

There were some locations referenceing a variable that was only
availabe with NXAGENT_ONSTART set

There was only one (commented) section using it.

This better reflects its purpose: Tell listeners we are ready.

They belong there...

Make them defines. Also determine the screen depth dynamically.

We either use a solid black or a white background and no backround
pixmap. So nxagentRootTileWindow is always empty and we can drop all
stuff around it.

remove nxagentSplashCount, too, since it is no longer checked anywhere.

nobody was checking it.

The splash window is only shown in x2go mode. In nxagent mode the
splash window was also shown, but empty (and thus invisible). And the
code waited for the splash window to disappear. Fix this by skipping
_all_ the splash stuff in nxagent mode.

Before there was no way of getting a white background despite having
the approriate code.

Backport of this commit:

  commit cb0a565d2b2cf8823abbd77b4426cc2237731dc1
  Author: Daniel Stone <daniel@fooishbar.org>
  Date:   Fri Aug 18 17:04:48 2006 +0300

    dix: add whiteroot flag
    Add a -wr option to use a white root window, and use a BackPixel rather
    than BackPixmap for both white and black root windows.

Fixes ArcticaProject/nx-libs#832