This is an extension for Tk800.* which will load a Photo image
from a snapshot of an X window specified by the -data option.
The window must exist and be visible. Because the code allows
you to capture windows not owned by Tk it does not attempt to
enforce this. If you are capturing one of Tks windows then
If window is mapped, but obscured by other windows then what is captured is the
rectangle the window would occupy. This can be considered a feature.
For Tk-owned windows $w->raise can used to bring window forward.
Once the Photo is loaded it can be saved using
$image->write(-format => ...) using any of formats which support