Added Loaders to generate texture at launch

Can be use to generate texture that fit the screen constraints on every device...

```java
manager.load("cachedResponsiveTexture", ProxyTexture.class, 
        // load texture from cached (as PNG) version
        new ProxyTextureLoader.ProxyTextureParameter(
                // or generate it with another loader if no cached version exists
                new FreePaintTextureLoader.FreePaintTextureParameter(
                    new FreePaintTextureLoader.Painter() {
                        @Override
                        public void draw(final Gdx2DPixmap pixmap) {
                            pixmap.fillRect(...);
                            // utility method of FreePaintTextureLoader.Painter
                            applyNoise(pixmap);
                            pixmap.fillCircle(...);
                            // anything else
                        }
                    },
                    Gdx.graphics.getWidth(), 
                    Gdx.graphics.getHeight()),
                FreePaintTexture.class
		)
);
```