bitmaptools not available for non-native CircuitPython (Blinka)

[bablokb]

Using this lib running with Blinka, the import of bitmaptools throws an error since this a native library not provided by Blinka. This was introduced with #76 while adding stroke support for Polygon.

A fix should be simple: bitmaptools is only used for a single function which can be replaced with a pure Python implementation in case bitmaptools is not available. Which might also be the case for native builds.