Fix datashader instability by disabling plot padding for Plotly RGB elements.

[jonmmease]

The introduction of plot padding last release broke the default behavior of using datashade with the plotly backend in the notebook. It resulted in the plot continually zooming out and crashing the kernel.

This simple (and I think appropriate) fix is to disable padding, by default, for the RGB elements with the plotly backend. Even apart from datashader, I think zero padding is a better default for RGB images as it's a bit awkward for there to be an empty border around them.

[jbednar]

Looks to me like this was just an oversight, because the Matplotlib and Bokeh backends already don't have padding for RGB or Image or Raster elements. I assume the same change should be made to Image and Raster?

[jonmmease]

Thanks, that makes sense. Done in 67fcb26