add use cases for benching script

[millzpaugh]

We'll want to add a series of use cases to bench composite, similar to vtquery. Here's the example vtquery script.

[mapsam]

This will be a great case for mvt-fixtures real-world tiles. Some that come to mind immediately (we'll likely think of more as we work on adding test cases):

• single tile in/out (just as a round trip confirmation)
• two different tiles at the same zoom level, zero buffer
• two different tiles at the same zoom level, with buffer
• two different tiles from different zoom levels (separated by one zoom), zero buffer
• two different tiles from different zoom levels (separated by more than one zoom), zero buffer
• two different tiles from different zoom levels (separated by one zoom), with buffer
• two different tiles from different zoom levels (separated by more than one zoom), with buffer
• tiles completely made of points
• tiles completely made of linestrings
• tiles completely made of polygons
• output compressed tile
• input two compressed tiles
• test overzooming on input with different layers e.g polygons-1 and polygons-2 to trigger some real work in node-mapnik

[millzpaugh]

All benching use cases have been added in #41 & 85821bb