Image RGBA downsample

This is still a WIP. I am fairly certain that the downsampling is working properly—[here is](http://nbviewer.ipython.org/urls/gist.githubusercontent.com/kdodia/47f4d9f829320fddbdac/raw/6ef780afb6f12ef16f5e7eb4b4a2e80ea0fbdafe/downsampling_rgba_poc.ipynb) a quick notebook that demonstrates the behavior—but I cannot identify the bug that prevents proper rendering.

It's probably something really simple too.

To test:
`./bokeh-server -D remotedata`
and then
`python examples/plotting/server/remote_image_rgba.py`