This patch adds a select_region option to the xcbgrab input device.
If set to 1, the user will be prompted to select the grabbing area
graphically by clicking and dragging. A rectangle will be drawn to
mark the grabbing area. A single click with no dragging will select
the whole screen. The option overwrites the video_size, grab_x, and
grab_y options if set by the user.
For testing, just set the select_region option as follows:
The drawing happens directly on the root window using standard rubber
banding techniques, so it is very efficient and doesn't depend on any
X extensions or compositors.