Reset Texture's cached GL SamplerState when device is reset

The cached sampler state was being persisted between device resets. After a context lost event, the GL sampler states are reset to default but the cached state was not. This meant that if your game used a single non-default sampler state, after a device reset it would never get applied.

Reproducing the problem is simple. Create a default game for Android and load a single texture that is smaller than the screen size into a `Texture2D tex` member. Then add the following drawing code. After a resume (run app, hit Home button, switch back to app), you will notice that the PointClamp sampler state is no longer being applied (texture looks smooth instead of blocky).

            spriteBatch.Begin(SpriteSortMode.Deferred, BlendState.AlphaBlend, SamplerState.PointClamp, DepthStencilState.Default, RasterizerState.CullNone);
            Rectangle rect =  new Rectangle(0, 0, Window.Width, Window.Height);
            spriteBatch.Draw(tex, rect, null, Color.White, 0.0f, Vector2.Zero, SpriteEffects.None, 0.0f);
            spriteBatch.End();

Codeplex discussion: http://monogame.codeplex.com/discussions/428652