compute_reproject_roi incorrect behaviour for non-overlapping geoboxes

[Kirill888]

Expected behaviour

When src and dst do no overlap I expect both src_roi and dst_roi to be empty on output. Seems like in some cases one can get empty src_roi while dst_roi isn't empty.

See user report here: opendatacube/datacube-stats#173

[Kirill888]

Problem is in this function:

datacube-core/datacube/utils/geometry/tools.py

Lines 600 to 607 in ac2866b

	def compute_roi(src, dst, tr, pts_per_side, padding, align):
	XY = np.vstack(tr.back(gbox_boundary(dst, pts_per_side)))
	roi_src = roi_from_points(XY, src.shape, padding, align=align)
	# project src roi back into dst and compute roi from that
	xy = np.vstack(tr(roi_boundary(roi_src, pts_per_side)))
	roi_dst = roi_from_points(xy, dst.shape, padding=0) # no need to add padding twice
	return (roi_src, roi_dst)

On line 602 roi_src can be empty, say it's a vertical line with zero width, but non-zero height. After projecting back to dst coordinate reference (line 605) vertical line becomes a slightly angled line, and so bounding box of that line is no longer zero width. So this explains how empty source roi can still result in non-empty dst roi.

The fix is to short circuit computation soon after line 602 and set dst_roi to some empty value like r_[0:0,0:0].