Fix text rendering in WPF

Text rendered in the WPF version seems blurry. If ```SetTransparentPainting``` in ```ManagedCefBrowserAdapter``` is commented out the rendered text looks the same as in the WinForms version (and exactly the same as rendered in Chrome).

This is the standard CefSharp welcome page enlarged a bit:
![previous_zoom](https://cloud.githubusercontent.com/assets/7759875/3676464/dcacf0aa-1288-11e4-94f4-7e3e12f747bc.PNG)

The same part of the welcome page after the change:
![cleartype_zoom](https://cloud.githubusercontent.com/assets/7759875/3676465/dcafae44-1288-11e4-9eb6-62e27377d0bc.PNG)