Make rasterization improvements

[drewbo]

The current rasterization code for segmentation labels doesn't support any additional options. For starters, it would be nice if there were an option to buffer Points and LineStrings so that they had a width greater than 1 pixel.

Proposed syntax would look like this in config.json:

"classes": [
    { "name": "Roads", "filter": ["has", "highway"], "buffer": 3 }
]

where 3 is the number of pixels to buffer the geometry by before rasterizing.