Fix grabbing for multi-screen configurations

The old version always uses the primary screen to capture the image. This
leads to problems for some variants of multi-screen configurations. For
example, my system uses 2 displays, the primary screen is on the right,
but the "zero" of cursor position corresponds to the top-left corner of
the left screen. This leads to the grabbing of some "garbage" when moving
the cursor on the right (primary) screen, and to the grabbing of the right
screen content when moving the cursor on the left screen -- looks like
madness :).

The new version selects the correct screen corresponding to the cursor
position and also uses relative (to the selected screen) coordinates
instead of absolute ones to grabbing the image.

(cherry picked from commit d8e6757b)