Fix geojson creation with ring-shaped masks

[leop-gne]

When a mask has a ring-shaped object in it, such as a pen mark that forms a closed loop, the current geojson code will create a filled disk in the geojson. Additionally, the use of cv2 means the original mask is not recoverable from the resulting geojson. This is a proposed fix, though I've only implemented it in 01_WSI_inference_OPENSLIDE_QC and haven't updated the dependencies since I'm not certain which GeoPandas and shapely versions work with the current dependencies.

Here's an example of a ring-shaped object and the mask created by the current method and the proposed method.

The new method displays the polygon as being offset by half a pixel from the original mask, but this is solely a display effect from the convention of whether a vertex designates the bottom right corner of a pixel or the center of the pixel at display time. The original mask is recoverable with rasterio.features.rasterize: