Tool for overzooming individual tiles (#121)

* Starting work on overzooming

* Factor clipping out of geometry.cpp to simplify linkage

* Pull out more geometry functions into now-badly-named clip.cpp

* Not surprisingly, there is a bug

* Found the bug

* Make indent

* Pass attributes through

* Make formatting more consistent

* Fix typos in comments

* Fix the typos better

* Add tippecanoe-overzoom to the install list

* Give overzoom a predictable exit status

* Forgot to translate to and from polygon ring closepaths

* Fix geometry collapse at z21

* Add attribute stripping; don't generate layers if they have no features

* Add docs for tippecanoe-overzoom (as it will be, not as it is)

* Change overzoom to accept input and output files as arguments

* Working on overzooming tests

* Oops

* Hook up and test the detail and buffer options, and the empty-tile case

* Also test attribute stripping

* Fix error message

* Update changelog and version

geometry.hpp

@@ -67,6 +67,7 @@ drawvec clean_or_clip_poly(drawvec &geom, int z, int buffer, bool clip);
 drawvec simple_clip_poly(drawvec &geom, int z, int buffer);
 drawvec close_poly(drawvec &geom);
 drawvec reduce_tiny_poly(drawvec &geom, int z, int detail, bool *reduced, double *accum_area, serial_feature *this_feature, serial_feature *tiny_feature);
+int clip(double *x0, double *y0, double *x1, double *y1, double xmin, double ymin, double xmax, double ymax);
 drawvec clip_lines(drawvec &geom, int z, long long buffer);
 drawvec stairstep(drawvec &geom, int z, int detail);
 bool point_within_tile(long long x, long long y, int z);