sample: Add sample that uses only cairo and writes image files

The sample writes the same PNG file over and over, with some pauses in
between iterations. Some indicative numbers are written to the console,
to help track performance and memory usage.

Feel free to add more elaborate drawing to this sample, to exercise more
of the cairo API.
cairo: Throw ObjectDisposedException after an object has been disposed

Potentially all these IDisposable classes could be used after being
disposed, which would result in native crashes. We now do an explicit
check and throw an exception in managed land when the object has been
disposed.

This is particularly useful because:
 a/ the native crashes are quite obscure, there no indication that
you're using a disposed object
 b/ Gtk# is passing Context instances to user methods, and disposes them
when the method returns. So if the user code keeps a reference to the
Context, there a good chance it will try to use it after it's disposed.

Other changes in this patch include:
 * Renaming a parameter to be more consistent with the other subsequent
ctor called.
 * Replacing implementation of some [Obsolete()] methods with the call
to the methods they were replaced with, to avoid redundancy and the
need for more CheckDisposed() calls than necessary.
 * Throw ArgumentException when receiving an IntPtr.Zero as a handle,
as a way to protect ourselves from wrapping invalid native pointers,
and throwing ObjectDisposedExceptions because the object was invalid in
the first place.

Signed-off-by: Bertrand Lorentz <bertrand.lorentz@gmail.com>