openslide.deepzoom: Fix longstanding overlap calculation bug

We were subtracting the top-left overlap in the wrong coordinate plane.
This only mattered for formats with perfect tile alignment and without
exact power-of-two downsamples, so was probably only relevant for Aperio.

Author: bgilbert

openslide/deepzoom.py

@@ -158,7 +158,7 @@ def _get_tile_info(self, dz_level, t_location):
 
         # Obtain the region coordinates
         z_location = [self._z_from_t(t) for t in t_location]
-        l_location = [self._l_from_z(dz_level, z) - z_tl
+        l_location = [self._l_from_z(dz_level, z - z_tl)
                     for z, z_tl in zip(z_location, z_overlap_tl)]
         # Round location down and size up
         l0_location = tuple(int(self._l0_from_l(slide_level, l))