made it possible to call pl.add_box() without needing to specify the axis

[MeyerBender]

fig, ax = plt.subplots(1, 2, figsize=(10, 5), dpi=200)
_ = ds.pp['DAPI'].pl.add_box([1500, 2000], [1500, 2000]).pl.show(ax=ax[0])
_ = ds.pp['DAPI'].pl.add_box([1500, 2000], [1500, 2000]).pl.show(ax=ax[1])

This code previously did not show the box for the first plot, the workaround was to specify ax=ax[0] in pl.add_box() as well. Now, rendering of boxes is deferred to the pl.imshow() function, so that all boxes are rendered at the very end of the rendering pipeline.