Fix combination of size and colour in geom_point

So far, `size` was handled over one trace (PR #108 for reference).
One layer in ggplot2 can correspond to multiple traces in plotly, typically when you make use of the `colour` aesthetics (see additional example in `test-ggplot-size.R`).
I applied the same idea as I once did for the x-range: You want to keep the global range (as opposed to per-trace) and scale accordingly within each trace.
/cc @pedrodz