The rectangle of an annotation is cached after it was painted. If just a
part of an editor was updated all annotations were removed but only
redrawn annotations were added to the cache again.

This behavior is replaced by removing annotations that are not visible
and those which got redrawn. So annotations that are still visible but
were not redrawn are kept in the cache.

Change-Id: I9246f1347b8f795284fb4fc9aabe11f251d16c25
Reviewed-by: Christian Stenger <christian.stenger@qt.io>