Wpf/WinForms: Fix initial draw of viewport in some cases.

[cwensley]

• Only subscribe to Paint to update the view (like before)
• Don't override Eto's Control.Shown or SizeChanged event registration, it caused the view to be redrawn many multiple times (and broke those events).
• Update the context during OnPaint() instead of calling OnResize to do the same as it has unintended side effects e.g. causes the view not to show initially.
• Set the background of the WPF control to ControlBrush so it doesn't show unpainted areas when resizing.