Draw points conditionally

With this fix, _drawPoints() will run only for non-empty tiles (More efficient for maps with less data, and slightly less efficient for maps with more data).

Removed variable declarations at the end of _draw().
Implicit conditional for loop

This pull request is optimal: instead of an if statement, have a regular for loop that has the condition `i < coordinates.length`.  Eliminating the _drawPoints function, and adding a comment, is probably all you need for clarity-sake.  Passing the coordinates[i] object once to _tilePoint(), instead of passing an array of object values simplifies the transformation from what quadtree gives you and what you need.  This should perform better.
Extract a _drawPoints function
Change license to AGPL