Return the position of previous text property or overlay 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.
In a string, scan runs to the start of the string.
In a buffer, it runs to (point-min), and the value cannot be less than that.
The property values are compared with `eq'.
If the property is constant all the way to the start of OBJECT, return the
first valid position in OBJECT.
If the optional fourth argument LIMIT is non-nil, don't search back past
position LIMIT; return LIMIT if nothing is found before reaching LIMIT. (fn POSITION PROP &optional OBJECT LIMIT)