[vtkwriter] detect whether `bind`/`unbind` are present instead of `localContext`
Currently the VTKWriter detects whether a function provides a method
localContext
to find out whether it may call bind
and unbind
.
It will never actually call localContext
.
This behavior is quite surprising. To fix the issue, this patch actually
checks for the methods that are actually called, i.e., bind
and unbind
.