3D depth-shading fix

Previously, a slightly buggy method of estimating the visual "depth" of 3D items could lead to sudden and unexpected changes in transparency as the plot orientation changed.

Now, the behavior has been made smooth and predictable. A new parameter depthshade_minalpha has also been added to allow users to set the minimum transparency level. Depth-shading is an option for Patch3DCollections and Path3DCollections, including 3D scatter plots.

The default values for depthshade and depthshade_minalpha are now also controlled via rcParams, with values of True and 0.3 respectively.

A simple example:

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=0.3,
)
ax.view_init(elev=10, azim=-150, roll=0)

plt.show()

(Source code, 2x.png, png)