Commit the state change of the element and proceed to the next pending state if any.
This function is used by elements that do asynchronous state changes. The core will normally call this method automatically when an element returned SUCCESS from the state change function.
If after calling this method the element still has not reached the pending state, the next state change is performed.
This method is used internally and should normally not be called by plugins or applications.
This function must be called with STATE_LOCK held.
a Element to continue the state change of.
The previous state return value