Group opacity disregarded in rasterize

Blending mode and opacity settings are lost when rasterizing a group. It's obviously unneccesary for the blending modes, but opacity information can be stored in the rendered bitmap. 

This also provides quite a large performance boost over setting the opacity on the `Raster` object - no need to render to a temporary canvas in each draw cycle. 

The simplest solution I found is to use the static `Item.draw` in the `rasterize` function, which handles these settings.