don't create infinitely many boxes for zero or negative height symbols

line-height or padding can make the height of a shaped object <= 0.
In these cases either create no boxes, or create boxes with a minimum
height to avoid creating too many of them.

-js: 9e78c02d8b22b60a08fdb28fd29eef1642f62d94

src/mbgl/text/collision_feature.cpp

@@ -15,9 +15,13 @@ CollisionFeature::CollisionFeature(const std::vector<Coordinate> &line, const An
     const float x2 = right * boxScale + padding;
 
     if (alongLine) {
-        const float height = y2 - y1;
+        float height = y2 - y1;
         const float length = x2 - x1;
 
+        if (height <= 0.0f) return;
+
+        height = std::max(10.0f * boxScale, height);
+
         bboxifyLabel(line, anchor, length, height);
     } else {
         boxes.emplace_back(anchor, x1, y1, x2, y2, std::numeric_limits<float>::infinity());