#69 Allow easy, dynamic vncviewer screen grab on unix

[S.Fourmanoit]

This allows external programs on a same machine than a
X11 vncviewer instance from the tightvnc project to
have easy access to its shared memory display XImage
when the MIT-SHM extension is used, as well as to the
latest damages that occurred.

The advantage over "classic" screen grab, besides
speed, is the permanent availability of such an Image,
while the drawable content will not be available as
soon as it is not in sight; this can be used to
implement all sort of nice overlay effects or snapshots
reliably, at a fraction of the processing cost of
XComposite.

Impact on resources is very minimal, if ever noticeable.

Along with the patch is a standalone test program that
will dump such an image from a vncviewer to a file.

[S.Fourmanoit]

The patch

[S.Fourmanoit]

test program