Checks if @stream is actually pollable. Some classes may implement
#GPollableInputStream but have only certain instances of that class
be pollable. If this method returns %FALSE, then the behavior of
other #GPollableInputStream 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 #GPollableInputStream but have only certain instances of that class be pollable. If this method returns %FALSE, then the behavior of other #GPollableInputStream 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.