mplot3d failed to clip bar charts if zlim is set.

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### Bug report

**Bug summary**

mplot3d failed to clip bar charts if zlim is set.

**Code for reproduction**

```python
# Adapter from http://matplotlib.org/2.0.2/examples/mplot3d/bars3d_demo.html
from mpl_toolkits.mplot3d import Axes3D
import matplotlib.pyplot as plt
import numpy as np

fig = plt.figure()
ax = fig.add_subplot(111, projection='3d')
for c, z in zip(['r', 'g', 'b', 'y'], [30, 20, 10, 0]):
xs = np.arange(20)
ys = np.random.rand(20)

# You can provide either a single color or an array. To demonstrate this,
# the first bar of each set will be colored cyan.
cs = [c] * len(xs)
cs[0] = 'c'
ax.bar(xs, ys, zs=z, zdir='y', color=cs, alpha=0.8)

ax.set_xlabel('X')
ax.set_ylabel('Y')
ax.set_zlabel('Z')

# Set zlim3d
ax.set_zlim3d(bottom=0.2, top=1)

plt.show()
```

**Actual outcome**


**Expected outcome**
The z-axis would be clipped according to the zlim_3d parameters properly

**Matplotlib version**
* Operating System: Windows 10 1703
* Matplotlib Version: 2.0.2
* Python Version: 3.6
* Jupyter Version (if applicable):
* Other Libraries:

Matplotlib installed from pip.

Use the supplied Python reproduction as the entry point, focusing on mpl_toolkits.mplot3d Axes3D.bar and ax.set_zlim3d. Compare the rendered bars with the expected z-axis limits, and consider the work complete when bars extending outside the limits are clipped correctly.

Written by the indexing model from the issue text.