Stats use default_aes prior to computation

[teunbrand]

This PR aims to fix #3860.

Briefly, it makes a Stat$use_defaults() method that evaluates Stat$default_aes prior to Stat$setup_data().
This is convenient for filling in optional aesthetics, like weight.

[teunbrand]

Also came across this:

ggplot2/R/stat-density-2d.R

Line 124 in d31c051

default_aes = aes(colour = "#3366FF", size = 0.5),

Where it seems weird to me that the stat declares aesthetics for the geom.