Add empty Polygon class
Add vertex and element buffers to the Polygon class
Add vertex and index lists to the Polygon class
Make the Polygon class drawable

It doesn't work properly yet though. A link needs to be made between the
vertex data and shaders. And we don't have shaders yet.
Make the Polygon class IDisposable
Fix the polygon class not being thread-safe, nor using the dummy context
Make the link between the vertices in the Polygon class and its effect
Modify the sample to showcase the new Polygon class
Minor cleanup
Fix crash on Nvidia cards

- Restore the old context after the current one is deactivated

- Fix dummy context activation and deactivation in some places

- Fix dummy context creation order

- Get rid of threaded windows, apparently it's impossible to get it to
  work properly
Merge branch 'feature/polygons' into develop