Return the position of next text property or overlay change.
This scans characters forward in the current buffer from POSITION till
it finds a change in some text property, or the beginning or end of an
overlay, and returns the position of that.
If none is found up to (point-max), the function returns (point-max).
If the optional second argument LIMIT is non-nil, don't search
past position LIMIT; return LIMIT if nothing is found before LIMIT.
LIMIT is a no-op if it is greater than (point-max). (fn POSITION &optional LIMIT)