Hollow markers are not transparent

Hello,

When I tried to plot hollow markers on the map, the marker is actually filled, not transparent. See this map below:


Below is the code to reproduce this map:

```python
import matplotlib.pyplot as plt
from mpl_toolkits.basemap import Basemap

lon_ll = -124.5 # lower left corner, longitude
lon_ur = -105.2 # upper right corner, longitude
lat_ll = 32.0 # lower left corner, latitude
lat_ur = 42.0 # upper right corner, latitude

f = plt.figure(dpi=100)
ax = plt.axes()

m = Basemap(llcrnrlon=lon_ll,llcrnrlat=lat_ll,
urcrnrlon=lon_ur,urcrnrlat=lat_ur,\
rsphere=(6378137.00,6356752.3142),\
resolution='c',projection='merc',\
lat_0=(lat_ll+lat_ur)/2,\
lon_0=(lon_ll+lon_ur)/2,lat_ts=20.,
epsg = 3310)

m.arcgisimage(service='World_Physical_Map', xpixels=120, epsg=3310, verbose=False)

m.drawstates()

LA = (34.05, -118.25)
LV = (36.175, -115.136389)
SF = (37.783333, -122.416667)
PH = (33.45, -112.066667)

ms = 15
alpha = 0.75

m.plot(*m(LV[1], LV[0]), marker='s', ms=ms,
ls='none', mew=1.25,c='blue', fillstyle='none', alpha=alpha)
m.plot(*m(LA[1], LA[0]), marker='o', ms=ms,
ls='none', mew=1.25,c='red', fillstyle='right', alpha=alpha)
m.plot(*m(PH[1], PH[0]), marker='o', ms=ms,
ls='none', mew=1.25,c='green', fillstyle='full', alpha=alpha)
m.plot(*m(SF[1], SF[0]), marker='o', ms=ms,
ls='none', mew=1.25,c='cyan', fillstyle='left', alpha=alpha,
markerfacecoloralt='white')

horiz_offset = 0.6 # unit: degrees
plt.text(*m(LA[1] + horiz_offset, LA[0]), 'Los Angeles')
plt.text(*m(LV[1] + horiz_offset, LV[0]), 'Las Vegas')
plt.text(*m(SF[1] + horiz_offset, SF[0]), 'San Francisco')
plt.text(*m(PH[1] + horiz_offset, PH[0]), 'Phoenix')
```

The markers for Los Angeles and Las Vegas should have been hollow (on the left side for LA, and fully for LV), but they are not. For half-full markers, such as the marker for San Francisco, I can set the `markerfacecoloralt` to other colors, but not for fully open markers.

- Operating system: Windows 10
- Python version: 3.6.6
- Basemap version: both 1.1.0 and 1.2.0 have this issue
- matplotlib version: 2.2.2

Note that I used to *not* have this issue some time ago, with older versions of matplotlib (I don't remember which).

Start by running the supplied Python reproduction with the listed Basemap and matplotlib versions, focusing on the m.plot entry point and the hollow, half-filled, and full markers. Compare the rendered output with the expected transparency described in the issue, then verify that hollow markers render unfilled while the other marker styles remain unchanged.

Written by the indexing model from the issue text.