Fix missing Patch3DCollection._z_markers_idx

The issue is introduced by commit f888285: attribute
`Patch3DCollection._z_markers_idx` is used within
`_maybe_depth_shade_and_sort_colors()` function, but is never assigned.

The same changes were done in the `Path3DCollection` class, but there
attribute is set correctly within `set_3d_properties()` function.
Assuming this is the expected behaviour, this commit also sets
`_z_markers_idx` in `Patch3DCollection.set_3d_properties()`

Author: slavoutich

lib/mpl_toolkits/mplot3d/art3d.py

@@ -462,6 +462,7 @@ def set_3d_properties(self, zs, zdir):
             xs = []
             ys = []
         self._offsets3d = juggle_axes(xs, ys, np.atleast_1d(zs), zdir)
+        self._z_markers_idx = slice(-1)
         self._vzs = None
         self.stale = True