Layers.capture doesn't respect images alpha value

When capturing a layer, it renders the sub-layers full opaque, even if they have an alpha value != 1

I didn't test this fully, I'm not sure if (or how to) I have to restore the alpha to the current value, something like

```
float oldAlpha = canvas.alpha(); // method doesn't exist
canvas.setAlpha(il.alpha());
canvas.drawImage(il.image(), 0, 0);
canvas.setAlpha(oldAlpha);
```