Images drawn by imagesc with background transparency lose bg transparency on export

A sort of minimum working example,

``` Matlab
I = imread('file.jpg');
alpha = I(:,:,1) > 235 & I(:,:,2) > 235 & I(:,:,3) > 235; %white->transparent
pic = imagesc(I);
pic.AlphaData = ~alpha;
export_fig(fig,output,'-nocrop','-transparent')
```

The result will have the original white background, rather than the transparent background hacked by setting the alphadata property.