make rasterize part of the raster render type

From voice with @kkaefer yesterday we should make rasterizing and blurring vector layers part of the raster render type. This will let us rasterize multiple layers together (performance) and treat rasterized layers just like any raster tiles (cross-fading, opacity).

In this example it would draw the full_shadow and medium_shadow into raster tiles together, and then draw them to the screen at 0.5 opacity.
```js
{
    "id": "hillshades",
    "render": {
        "type": "raster",
        "raster-size": 512,
        "raster-blur": 2,
    },                                          
    "style": {  
        "raster-opacity": 0.5
    },                              
    "layers": [     
        { "id": "full_shadow", "render": {}, ... },
        { "id": "medium_shadow", "render": {}, ... },
        ...                         
    ]   
}
```

No migration is needed because none of the styles being migrated have this.

@kkaefer @lbud @mourner