Fix accidental loss (at all zooms) of features with an explicit minzoom

[e-n-f]

As reported in #219, versions of tippecanoe since 2.43.0 have accidentally dropped all features that specified an explicit minzoom with "tippecanoe":{"minzoom":nnn}. This PR adds an else clause to retain these features in the specified zoom range.

The test file is the Natural Earth 110m populated places, with the scalerank used as the minzoom. Each feature contains an object like this to specify its minzoom:

{"type":"Feature","tippecanoe":{"minzoom":8},"properties":{…},…}

The unrelated change to mbtiles.cpp and joined_reordered.json is to round the tileset bounding boxes and centers to six digits to avoid test failures on some platforms from slightly different floating point behavior.

[springmeyer]

Change looks good 👍🏼

The rounding fix ensures all tests pass for me locally with Apple clang version 15.0.0 (clang-1500.1.0.2.5) on an M2 Mac.

I also confirmed that the new test you added looks good: it correctly catches the problem. If I comment out the fix:

diff --git a/tile.cpp b/tile.cpp
index e8ce1ce..a166939 100644
--- a/tile.cpp
+++ b/tile.cpp
@@ -1179,7 +1179,7 @@ static serial_feature next_feature(decompressor *geoms, std::atomic<long long> *
                                sf.dropped = FEATURE_DROPPED;
                        }
                } else {
-                       sf.dropped = FEATURE_KEPT;
+                       //sf.dropped = FEATURE_KEPT;
                }
 
                // Remove nulls, now that the expression evaluation filter has run

Then I see the new test fail, as expected:

tests/ne_110m_populated_places-minzoom/out/-yNAME_-ySCALERANK_-z5.json.check.out tests/ne_110m_populated_places-minzoom/out/-yNAME_-ySCALERANK_-z5.json differ: char 195, line 4
make: *** [tests/ne_110m_populated_places-minzoom/out/-yNAME_-ySCALERANK_-z5.json.check] Error 1