Artifact, asymetric square in ImagePlaneWidget

Hi,
For data source I got scalar field (3d) and draw ImagePlaneWidget (for clarity I set texture_interplate = False, also reslice_interpolate = nearest_neighbor, also added GridPlane to view grid):

```
s = np.zeros((11,11,3), )
s[1,5,1] = .5
s[3,6,1] = .5
s[5,7,1] = .5

s[5, 1,1] = .5
s[6, 3,1] = .5
s[7, 5,1] = .5

src = self.scene.mlab.pipeline.scalar_field(s)
viz = self.scene.mlab.pipeline.image_plane_widget(src, plane_orientation='z_axes', slice_index=1, )
```
What I got: not all red squares are simmetric.
What I expect: all symmetric red squares.

I want ImagePlaneWidget element (voxel) be same dimensions as grid shape (symmetric square). But I got different shapes: square - if x or y coordinate is odd, and rectangle - if x or y is even.
Where problem could be?

Many thanks!