rgba formats

Mike (and others),

The standard rgba format in mpl has been a sequence of 4 doubles, but these need to be converted to uint8 before they can be used by Agg, at least. Therefore I added the ability for ScalarMappable.to_rgba to generate uint8 initially, avoiding the extra translation. (This can result in a substantial speedup.) I haven't looked into the other backends, but I am wondering whether, at least in the transform branch, it might make sense to do this more uniformly--that is, go straight to uint8. Or are you already doing this, Mike? Does one ever need more than 8-bits per channel of color resolution? Are all the backends in fact having to do this conversion from 4 doubles to 4 uint8 anyway?