Depth-shading fix and more depth-shading options

New options have been added which allow users to modify the behavior of

depth-shading while addressing a visual bug.

Previously, a slightly buggy method of estimating the "depth" of plotted

items could lead to sudden and unexpected changes in transparency as the

plot orientation changed.

Now, the behavior has been made smooth and predictable, and the user is

provided with three new options: whether to invert the shading, setting the

lowest acceptable alpha value (highest transparency), and whether to use

the old algorithm.

The default behavior visually matches the old algorithm: items that appear to be

"deeper" into the screen will become increasingly transparent (up to the now

user-defined limit). If the inversion option is used then items will start

at maximum transparency and become gradually opaque with increasing depth.

Note 1: depth-shading applies to Patch3DCollections and Path3DCollections,

including scatter plots.

Note 2: "depthshade=True" must still be used to enable depth-shading

A simple example:

.. plot::

import matplotlib.pyplot as plt

fig = plt.figure()

ax = fig.add_subplot(projection="3d")

X = [i for i in range(10)]

Y = [i for i in range(10)]

Z = [i for i in range(10)]

S = [(i + 1) * 400 for i in range(10)]

ax.scatter(

xs=X,

ys=Y,

zs=Z,

s=S,

depthshade=True,

depthshade_minalpha=0.1,

depthshade_inverted=True,

depthshade_legacy=True,

)

plt.show()