Distinguish between signed and unsigned in the temporary file
Trying to follow Vlad's suggestion and work from the top down.

Doesn't quite work yet, but seems like the right thing to do.
From the bottom up, indexing is a mess because of the buffering,
and includes unneeded areas in the bounding boxes of big features.
Don't introduce bogus geometries in clipped-away features.
Fix the major bug: forgetting to offset back to world coordinates
Remember to remove the closepath after clipping away a polygon
Use bounding box for quick accept/reject before detailed clipping.
Partition each tile into children instead of repeatedly clipping the parent.
No need to remove no-ops here unless we clipped
More useful progress indicator
Merge branch 'master' into topdown
Fix compiler warning
Make -Z0 work again for file minimum zoom
No need to split features into child tiles when processing the final zoom.
Remove unneeded old indexing code
Merge branch 'master' into topdown

Conflicts:
	geojson.c
Remove unneeded debug output
Keep the progress indicator progressing while working through big tiles
Oops. Do a lot less progress indicator I/O.
Fix failure to release temporary memory
Merge pull request #18 from mapbox/topdown

Work from the top down instead of from feature indices