Pyramid onlyempty

This branch removes the shortcut in the pyramid stream which tries to reuse a solid parent tile for its children without refetching the child tiles. Only affects solid tiles which contain content -- empty tiles are still skipped per usual.

---

**Explanation**

Suppose we are rendering a tile at the very edge of a polygon that is rectangular.

          |............
          |............
          +-----+......
     tile |.....|......
          |.....|......
          +-----+......
          |............
          |.polygon....

If this tile has a slight buffer the polygon will hang over the N/E/S edges and be flush with the W edge when clipped.

          ........
          +-----+.
     tile |.....|.
          |.....|.
          +-----+.
          ........

**Before**

This buffer (identified currently as "solid") was reused, resulting in four child tiles that look like this:

          ........   ........
          +-----+.   +-----+.
          |.....|.   |.....|.
          |.....|.   |.....|.
          +-----+.   +-----+.
          ........   ........
                             
          ........   ........
          +-----+.   +-----+.
          |.....|.   |.....|.
          |.....|.   |.....|.
          +-----+.   +-----+.
          ........   ........

When rendered with a fill + stroke you'd see an extra edge because the polygon is flush with the W edge of the vector tile:

          |......|......
          |......|......
          |......|......
          |......|......
          |......|......
          |......|......
          |......|......
          |......|......

**After**

Instead of repeating the parent buffer we read all the child tiles as normal. The two child tiles on the right should have a W edge buffer overhang:

          ........  .........
          +-----+.  .+-----+.
          |.....|.  .|.....|.
          |.....|.  .|.....|.
          +-----+.  .+-----+.
          ........  .........
                             
          ........  .........
          +-----+.  .+-----+.
          |.....|.  .|.....|.
          |.....|.  .|.....|.
          +-----+.  .+-----+.
          ........  .........

When these tiles are rendered with a slight line stroke you'll get this

          |.............
          |.............
          |.............
          |.............
          |.............
          |.............
          |.............
          |.............


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Potential future solutions are to make the upstream solid detection in `node-mapnik` more clever (e.g. look for square clipped polygons equidistant/outside the tile boundaries) but for now disable repeated use of tile buffers in pyramid scheme.

cc @springmeyer @makella