patch 9.2.0861: GTK4: bleed region updates in jumps

patch 9.2.0861: GTK4: bleed region updates in jumps

Problem:  In the GTK4 GUI (GtkSnapshot) the bleed region is only
          repainted when the text grid changes by a whole cell,
          so during a resize it updates in jumps of several pixels
          instead of following the window edge smoothly.
Solution: Redraw the draw area whenever the bleed values change
          (Foxe Chen).

closes: #20851

Signed-off-by: Foxe Chen <chen.foxe@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>