MacOSX backend supports 2x DPI images and MathTeX.

This is a bit of a hack to fix #1958.  Test with

<pre>
import numpy as np, pylab as pl
pl.imshow(np.arange(1000000).reshape(1000,1000) % 997)
pl.text(0,100,'$foo$')
</pre>

The behavior on figimage is possibly not correct?