MRG: Fix window size for screenshot on HiDPI

[larsoner]

On macOS with HiDPI mlab.screenshot is broken, only rendering 1/4 of the window (as expected by the 2x logical to physical pixel ratio for the monitor):

>>> from mayavi import mlab
mlab.>>> mlab.test_plot3d()
<mayavi.modules.surface.Surface object at 0x128adb728>
>>> img = mlab.screenshot()
>>> import matplotlib.pyplot as plt
>>> plt.imshow(img)
<matplotlib.image.AxesImage object at 0x133546ef0>
>>> plt.show()

On master:

On this PR:

Not 100% sure figure.scene.render_window.size is the right thing to use here, but it works.

[codecov-io]

Codecov Report

Merging #660 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #660   +/-   ##
=======================================
  Coverage   50.45%   50.45%           
=======================================
  Files         257      257           
  Lines       23328    23328           
  Branches     3181     3181           
=======================================
  Hits        11771    11771           
  Misses      10802    10802           
  Partials      755      755

Impacted Files	Coverage Δ
mayavi/tools/figure.py	47.79% <0%> (ø)	⬆️

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[prabhuramachandran]

Thanks! LGTM!

[larsoner]

FWIW I just noticed savefig does something different involving magnification:

https://github.com/enthought/mayavi/blob/master/mayavi/tools/figure.py#L234

Maybe the two approaches should be unified?

[prabhuramachandran]

Yes, that would be good. :)