Fix rendering with tileMargin on top and bottom border of map (#571)

Co-authored-by: Frank Stolle <f.stolle@hosting-agency.de>

src/serve_rendered.js

@@ -305,9 +305,11 @@ module.exports = {
           });
 
           if (z > 2 && tileMargin > 0) {
+            const [_, y] = mercator.px(params.center, z);
+            let yoffset = Math.max(Math.min(0, y - 128 - tileMargin), y + 128 + tileMargin - Math.pow(2, z + 8));
             image.extract({
               left: tileMargin * scale,
-              top: tileMargin * scale,
+              top: (tileMargin + yoffset) * scale,
               width: width * scale,
               height: height * scale
             });