OnResize logic isn't correct

[NvC-DmN-CH]

https://github.com/opentk/LearnOpenTK/blob/6825eba6a99e097236eb17b768f59c157e6dadde/Chapter1/2-HelloTriangle/Window.cs#L185

I set the window to (1280, 720) but the GL.Viewport is actually set to:

When I used ClientRectangle.Size.X and ClientRectangle.Size.Y it became proper:

[NogginBops]

The correct property is actually FramebufferSize, others will break on macOS. Will fix next time I do some work on these tutorials (if no one else has made a PR)

[NvC-DmN-CH]

Oh ok, thank you!