Make OpenGL vertex shader dirty on viewport change

This pull request fixes an offset bug on OpenGL platforms when changing the viewport size.

The OpenGL version of MonoGame uses an uniform named "posFixup" to offset any rendering by a fraction of the viewport size. This is required because DirectX coordinates refer to pixel centers while OpenGL coordinates refer to pixel corners.

Whenever the render target is changed, the vertex shader is marked dirty so that the posFixup uniform gets updated (the viewport changes whenever the render target is changed).

This change ensures the posFixup uniform is updated whenever the viewport changes, not only when it is changed through a render target change.

I wrote a testcase to reproduce & fix the bug, you can download it here: http://polyprograms.free.fr/tmp/BlurredSpriteBatch.zip

XNA version:
![BlurredSpriteBatch](https://f.cloud.github.com/assets/446986/61757/3fbbd092-5c7f-11e2-8dca-ab9a9ca4967c.png)

MonoGame version (without the fix) - text & sprite are offset / blurred:
![BlurredSpriteBatchMonoGame](https://f.cloud.github.com/assets/446986/61758/3fd2d440-5c7f-11e2-9101-436342885537.png)

MonoGame with fix version:
![BlurredSpriteBatchMonoGameWithFix](https://f.cloud.github.com/assets/446986/61759/3fe9b98a-5c7f-11e2-8962-0624f5e2ec7f.png)