BUG: Fetch current scale before use in roi methods

multiscales.images[-1] is not going to produce the expected results.

Explicitly fetch the current scale when getting the roi slice. I did not
test this, but hopefully it will yield more consistent results if the
viewer is exploring different resolutions.

Author: thewtex

itkwidgets/viewer.py

@@ -423,6 +423,8 @@ async def get_roi_image(self, scale: int = -1, name: str = 'Image') -> NgffImage
         :return: roi_image
         :rtype:  NgffImage
         """
+        if scale == -1:
+            scale = await self.get_current_scale()
         roi_slices = await self.get_roi_slice(scale)
         roi_region = await self.get_roi_region()
         if store := self.stores.get(name):
@@ -498,6 +500,8 @@ async def get_roi_slice(self, scale: int = -1):
         :return: roi_slice
         :rtype:  List[slice]
         """
+        if scale == -1:
+            scale = await self.get_current_scale()
         idxs = await self.viewer_rpc.itk_viewer.getCroppedIndexBounds(scale)
         x0, x1 = idxs['x']
         y0, y1 = idxs['y']