Add patterns to generated symbols and sprite

[bkeepers]

This adds patterns to the generated sprite.

Unfortunately the style spec does not have any mechanism for controlling the spacing of patterns (mapbox/mapbox-gl-js#4250). So the only solution right now is to pre-render the patterns with the desired spacing.

For linear patterns, this should only require expanding the SVG viewbox to add the spacing before rendering the sprite. For staggered patterns, this will require duplicating the symbol in the SVG with the stagger offset.

Many patterns also have min/max spacing. Using zoom expressions, it should be possible to use different images from the sprite at different zoom levels. For linear patterns, it should be possible to only include the symbol in the sprite once with the maximum spacing, and then add multiple entries in the sprite JSON with different spacing for each zoom. For staggered patterns, the SVG will need to be drawn at each zoom level, and the symbol will be duplicated in the sprite.

• Linear patterns
• Staggered patterns
• min/max spacing

cc #12