Data limits in scatter function

Updates the data limits in a scatter plot with just the (x,y) input points. Currently, the `scatter` function creates a collection of markers which then are allowed to update the data limits. This has unintended consequences as seen in issue #3059.
This doesn't make sense to me. What if transOffset is not transData? It seems like add_collection should be handling all this correctly, and if it isn't then that is what needs to be fixed, not the code in scatter.
My impression is that the problem is coming from the basic design of mpl, in which some variables are set, calculations are done, and then later changes in those variables are not taken into account.  The only way around it is a major redesign in which a full dependency relationship is established, and the evaluation occurs only at the last moment.