-r option and positioning of colorbar text

When I issue `-r300` option of a `pcolor`-ed graph, the colorbar text is positioned incorrectly.

Minimal example:

```Matlab
pcolor( rand(10,10) );
h = colorbar;
h.Title.String = 'x';
export_fig('test.png','-r300')
```

and the output looks like (notice `x` hovering around 0.25 in the colorbar):

On-screen, everything looks ok:

My current version is commit a0f8ec376d06

P.S. Thank you for the excellent work on this code, it's a lifesaver!