fix for platforms without bitmaptools

[bablokb]

This fixes #80

Tested with Blinka_Displayio_PyGameDisplay

[FoamyGuy]

I'd prefer not to add this fallback here and instead get bitmaptools implemented for Blinka_DisplayIO. I've recently begun working on just that on this branch: https://github.com/FoamyGuy/Adafruit_Blinka_Displayio/tree/bitmaptools

fill_region() and a handful of other methods are already implemented in that branch, if you're willing and interested feel free to give it a try as a way to get this library working without the need for the fallback logic that this PR adds.

I'll open a PR to Blinka_Displayio repo with it once I get a few more of the remaining functions implemented.

[bablokb]

I agree that adding bitmaptools to Blinka is the correct way to go. But until this is merged, the import of Polygon already causes a program to crash with an exception. So this PR is useful as a temporary workaround. This depends on how fast your code will be merged to Blinka.