Avoid bleeding of inset background outside of inset
As a consequence of fix
e64ea357 to ticket #10797, we draw a bit too
much of the inset background outside of the inset (visible for insets
with colored background). #10797 is a ticket that triggers when the
cursor has a width larger than 1.
This patch limits the problems in two respects
* nothing is done on the left, since the cursor width only expands on
the right.
* on the right, the extra width is limited to cursor width.
Fixes bug #11786.
(cherry picked from commit
eb92f74d1f7ae63112b9439058d746f7ff93e8f4)