Fix scalebar rendering issue with multiple axes

[DavidVadnais]

Fixes: #368

when a figure has more than one axes and drawmapscale(ax=ax0,...) is called while other axes (ax1) is up, scalebar is split

I have a test case for this but ill i try to narrow it down more. currently:

import matplotlib; matplotlib.use('Agg')
import matplotlib.pyplot as plt
from mpl_toolkits.basemap import Basemap

fig = plt.figure()
ax0 = fig.add_axes([0, 0, 1, 1])
fig.add_axes([.6, .6, .2, .2])  # becomes the "current" axes

m = Basemap(projection='lcc', width=3e6, height=3e6,
            lat_1=50, lat_0=47, lon_0=1, resolution=None)
rets = m.drawmapscale(-15, 47, -15, 47, 400, barstyle='fancy', ax=ax0)

bad = sum(r.axes is not ax0 for r in rets)
print(f"{bad}/{len(rets)} artists NOT on ax0") # if more then 0/N this is a failure.

[DavidVadnais]

code from issue to see bug:

import pylab as pl
from mpl_toolkits.basemap import Basemap
m = Basemap(width=3000e3,height=3000e3,resolution='l',
         projection='lcc',lat_1=50.,lat_0=47,lon_0=1.)

ax0=pl.axes()
ax1=pl.axes([.6,.5,.15,.15], facecolor='#ddeeff')
ax1.set_title('ax1 (should stay empty)', fontsize=7)
m.drawcoastlines(ax=ax0)
lon,lat=-15,47
lon0,lat0=lon,lat
scaleL=400
#pl.axes(ax0) #this will avoid the problem...
o=m.drawmapscale(ax=ax0,lon=lon,lat=lat,lon0=lon0,lat0=lat0,
                   length=scaleL,barstyle='fancy',fontcolor='r')

pl.show()

Before:

After: