Round coordinates instead of truncating them (#60)

* Round coordinates instead of truncating them

* Update all the tests for coordinate rounding changes

* Curses, integer division still truncates

* Fix tests

* Also round instead of shifting when looking for no-op linetos

* Should I worry that the same change for moveto doesn't change any tests?

* Also round instead of shifting when scaling down to maxzoom resolution

* Replace another explicit shift, for origin point

* Round instead of shift when writing clipped geometries to the next zoom

* Fix low-zoom gridding and smaller-than-a-pixel checks

* Don't guess an excessively large maxzoom when there is only one feature

* Add a test for guessing the maxzoom of a single point

* Explicitly sort by index if no other order distinguishes features

* Another affected test

main.cpp

@@ -2276,6 +2276,8 @@ std::pair<int, metadata> read_input(std::vector<source> &sources, char *fname, i
 			for (z = 0; z <= MAX_ZOOM; z++) {
 				unsigned xxx = 0, yyy = 0;
 				if (z != 0) {
+					// These are tile numbers, not pixels,
+					// so shift, not round
 					xxx = xx >> (32 - z);
 					yyy = yy >> (32 - z);
 				}