Figures upside-down in headless mode

When I use mayavi 4.3.0 (Enthought Canopy) in headless mode, figures saved using `mayavi.mlab.savefig` are upside-down. E.g.:

```
import numpy
from mayavi.mlab import *
s = numpy.abs(numpy.random.random((3, 3)))
barchart(s)
savefig('test.png')
```

if saved as "test.py" and running it using

```
python test.py
```

gives the correct figure


However, if I use

```
DISPLAY=:1 xvfb-run -f /tmp/xauthority-tmp --auto-servernum \
-s "-screen 0 1280x1024x24 -noreset" python test.py
```

the figure is upside-down:


The figures are not upside-down when I use mayavi 4.2.0 shipped with EPD-7.3.1, so it seems to be a problem with either mayavi or VTK.

CC @agramfort @dengemann @Eric89GXL