Avoid bleeding of inset background outside of inset

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.