Return the position of previous property change for a specific property.
Scans characters backward from POSITION till it finds
a change in the PROP property, then returns the position of the change.
If the optional third argument OBJECT is a buffer (or nil, which means
the current buffer), POSITION is a buffer position (integer or marker).
If OBJECT is a string, POSITION is a 0-based index into it.
The property values are compared with `eq'.
Return nil if the property is constant all the way to the start of OBJECT.
If the value is non-nil, it is a position less than POSITION, never equal.
If the optional fourth argument LIMIT is non-nil, don't search
back past position LIMIT; return LIMIT if nothing is found until LIMIT. (fn POSITION PROP &optional OBJECT LIMIT)