Add shader based blendmodes

Render an other texture onto the current texture with different blend modes (idea by @b2renger).

```java
PGraphics texture1;
PGraphics texture2;

fx.render(texture1)
        .brightnessContrast(0.1f, 1.0f)
        .blend(BlendMode.SCREEN, texture2)
        .compose(passResult);
```

Here a list of different blend modes:

http://www.northlite.net/ps/blend.htm