[matplotlib-devel] 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?

Eric