Celiagg backend doesn't save images correctly

[corranwebster]

From enable, when I call save to put the contents of a celiagg gc into a .png file, I get this:

where if I use the old agg backend (or indeed anything else), I get this:

which is what I would expect based on the colors, etc. I was using.

Based on a little experimentation, the channel shuffling doesn't seem to be needed

[rkern]

That method needs to check what self.pix_format before it does anything, since the pix_format is a user's choice and may not be rgba32 (or bgra32, which is what it is assuming incorrectly now).