Use make_color_tuples around hex_to_rgba

Deal with a mixture of list-like (mappings) and scalar (direct settings)
`facecolor` and `alpha` aesthetics. `make_color_tuples` knows how to
do this.
Update geom_rect

- geom_rect now uses collections to draw the rectangles. This is
  faster and clearer.
- fixed `scale_size.__init__`, the range argument is not a range
  for the scale in that it can be trainned, it is a range for the
  palette.
- Instance of `scale` should be able to initialize and set the
  scale parameters e.g limits.