black background on rasterized quadmesh in ps output

[tacaswell]

This reproduces the problem:

X, Y = meshgrid(linspace(0, 1, 2048), linspace(0, 1, 1024))
Y *= 2 + sin(linspace(0, 2*np.pi, 2048))
C = np.sin(X + Y)
pcolormesh(X, Y, C, rasterized=True)
gcf().savefig('test.eps')
gcf().savefig('test.png')

Saved as eps (converted the eps -> png for upload with inkscape)

saved as png (and matches screen):

My guess is that the agg layer renders the parts of the bounding box around the artist where the quad-mesh isn't as transparent and iirc postscript can't deal with the alpha channel. I hope this is an easy fix (change the background to (1, 1, 1, 0)?), but I don't know the guts of the rendering layer well enough to tell what to change.