Checks if stream is actually pollable. Some classes may implement
GPollableOutputStream but have only certain instances of that
class be pollable. If this method returns FALSE, then the behavior
of other GPollableOutputStream methods is undefined.
For any given stream, the value returned by this method is constant;
a stream cannot switch from pollable to non-pollable or vice versa.
Checks if stream is actually pollable. Some classes may implement GPollableOutputStream but have only certain instances of that class be pollable. If this method returns FALSE, then the behavior of other GPollableOutputStream methods is undefined.
For any given stream, the value returned by this method is constant; a stream cannot switch from pollable to non-pollable or vice versa.