Mask-based ROI data structure

[aloejhb]

Since many features, e.g. importing StarDist ROI mask and ROI mask transfromation using bUnwarpJ, are based on ROI mask images, it will be more efficient to represent each single ROI with their corresponding x-y location in the mask image.
The current ROI data structure is storing the boundary location of the ROI polygon. This cause unwanted change of ROI shape during transformation, import and export of ROI masks. In addition, transformation between polygon and mask is time-consuming when computing for many ROIs.
With this, it would be nice to change to a mask-based ROI data structure.