Update Util.cs

RenderTarget2D is volatile. This means when there is any change to screen resolution, the content will be lost. This is common when changing to full-screen, alt+tab, screensavers, and locking/unlocking the computer. We can counteract this by directly setting the content of Texture2D.
Update Util.cs

Remove unneeded casts.
Merge pull request #63 from BigRedPK/master

XNA RenderTarget2D Fix