PatchCollection edges incorrect when clip path is set

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### Bug report

**Bug summary**

Cartopy has a mostly-superfluous `GeoAxes.background_patch` which I'm trying to convert to the standard `Axes.patch`. By default, the `Axes.patch` is a `Rectangle`, which triggers a fast-path to set a clip box instead of a clip path. However, since most map boundaries are non-square, the full clip path code is used instead. When doing so, this causes some strange artifacts.

(Un)Fortunately, it can be reproduced with plain Matplotlib below.

**Code for reproduction**

```python
import matplotlib.pyplot as plt
import matplotlib.patches as mpatches
from matplotlib.collections import PatchCollection
from matplotlib.path import Path

pth = Path([[0, 0], [1, 0], [1, 1], [0, 1], [0, 1]],
[1, 2, 2, 2, 79])

fig, ax = plt.subplots(2, 2, sharex=True, sharey=True)
ax[0, 0].set_xlim(-1, 2)
ax[0, 0].set_ylim(-1, 2)

for a in ax[0]:
collection = a.add_collection(
PatchCollection([mpatches.PathPatch(pth)],
facecolor='C0', edgecolor='k'))

collection.set_clip_path(
Path([[0, 0], [1, 0], [1, 1], [0, 1], [0, 1]]),
a.transAxes)

for a in ax[1]:
patch = a.add_patch(mpatches.PathPatch(pth, facecolor='C0', edgecolor='k'))

patch.set_clip_path(
Path([[0, 0], [1, 0], [1, 1], [0, 1], [0, 1]]),
a.transAxes)

ax[0, 0].set_ylabel('PatchCollection')
ax[1, 0].set_ylabel('PathPatch')
ax[1, 0].set_xlabel('Default clip box')
ax[1, 1].set_xlabel('Custom clip path')

plt.savefig('test.png')
```

**Actual outcome**

In the bottom row is a plain `PathPatch`, and the top row is a `PatchCollection` containing the same `PathPatch`. The left column shows the default clipping (to the clip box of the `Axes`). The right column uses a clip path of a manually-specified 0-1 rectangle in `Axes` space, so it should be equivalent.

But the horizontal edges are either doubled over or the wrong width in the top-right case.


**Expected outcome**

All path edges should look the same.

**Matplotlib version**

* Operating system: Fedora 30
* Matplotlib version: 3.1.3 and master, 9d00ca839e5faf10257095ad1e2bfb5296549d47
* Matplotlib backend (`print(matplotlib.get_backend())`): TkAgg (probably only the Agg bit is relevant)
* Python version: 3.6.9 / 3.6.3

Start with PatchCollection.set_clip_path and compare its rendering path with the PathPatch case in the supplied reproduction. Run the example with the reported versions or a current checkout, then verify that the PatchCollection's clipped horizontal edges match the PathPatch edges and are not doubled or incorrectly sized.

Written by the indexing model from the issue text.