Fix SpriteEffect half pixel offset

[tomspilman]

Now that we've made SpriteEffect public we have to make sure it works.

Currently you can see it is always applying a half pixel offset... even on platforms that don't require it:

https://github.com/mono/MonoGame/blob/develop/MonoGame.Framework/Graphics/Effect/SpriteEffect.cs#L62

It should use the SpriteBatch.NeedsHalfPixelOffset flag like SpriteBatch does:
https://github.com/mono/MonoGame/blob/develop/MonoGame.Framework/Graphics/SpriteBatch.cs#L140

While we're at it we should add in a unit test for it since we need to ensure it works now.

[Verser]

@tomspilman Just checking, this seems to be complete but is marked as Open and HelpWanted. Should this be closed?

[tomspilman]

@Verser - Thanks... we just forgot to close this.