screenshot: Snap cursor to the center of the handle on press

screenshot: Snap cursor to the center of the handle on press

When dragging the screenshot area handle from the outer sides, it is
currently impossible to move them to the edges or corners of the screen,
as the cursor is constrained to the monitor.

Fix this by snapping the cursor to the center of the handle when starting
the resize operation.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/9113
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4158>