pngtosvg: Merge thresholding part in pngToBmp to sheettopng.py code to accommodate variable lighting in inputs

Related: #3

We should do this thresholding in sheettopng during the cropping process.

https://github.com/builtree/handwrite/blob/adca270f90e3ac5c3eae31eb1e49e05f3142bcd3/handwrite/pngtosvg.py#L69-L76