Fix crash when borders exceed item width/height in border image

While we protected against the the borders exceeding the size of
the source image when deciding whether to create a given patch,
we did not protect against the case where the target rectangle
is smaller than the borders. To fix this, we simply move the
calculation of the target rectangle up to before we create the
nodes, and check for isEmpty() before creating the nodes.

In addition, we did not properly handle changing the borders
dynamically. The subtree has to be rebuilt if the borders change
so that the source or target rectangles change.

Change-Id: Ia6a0df616ebbd0a32924de0b63fd48043027930a
Task-number: QTBUG-41338
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>