The MRenderView::updatePixels API has an annoying side-effect of showing a white outline. For example, calling it in the default way on a rectangle will show it like this:
There is an undocumented API available that can alleviate this problem. You can call the MRenderView::setDrawTileBoundary(false) method after startRender and it will disable Maya from drawing the outline.
Unfortunately, this API is currently marked for deprecation. But we have discussed this with our engineering team, and they agree it is an acceptable workaround for the time being.