add yolov8 mask support, improve mask processing speed by 4-5x

[mayrajeo]

Cleaner version of #888 . Adds support for instance segmentation with yolov8 and makes slicing instance segmentation results use COCO-style annotations. Haven't yet tested with oriented bounding boxes.

Known shortcomings:

• NMS/NMM/GREEDYNMM is still done with bounding boxes instead of polygons
• Not a problem of sahi, but yolov8 models can output multipart polygons even though yolov8 annotation format doesn't support those.