Fix white gap between image and border

Fixes #218. I have manually tested this with the 'qt4.image', 'qt4.qpainter', and 'wx.image' backends on OSX.

I seen elsewhere the center-pixel adjustment used here, but I am uncertain about the details of why it is needed. For example, the `ColorBar` does not need this adjustment. (Is there something wrong with the `kiva.agg.GraphicsContextArray` used in the `ImagePlot`?)

@rkern, would you mind reviewing this?
Is this true for all `GraphicsContexts`?