Gradients in ribbons

[teunbrand]

This PR aims to fix #4690.

Briefly, geom_ribbon() (and by extension geom_area()) didn't allow varying fill or alpha aesthetics within a group.
This PR encodes fill and/or alpha in a grid::linearGradient(), which allows these aesthetics to vary.

Demo for fill:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

ggplot(economics, aes(date, unemploy, fill = uempmed)) +
  geom_area(stat = "identity") +
  scale_fill_viridis_c()

Demo for alpha:

ggplot(economics) +
  aes(date, ymin = unemploy - 200, ymax = unemploy + 200, alpha = pop) +
  geom_ribbon()

^{Created on 2024-02-22 with reprex v2.1.0}

A few limitations:

• Using this in non-linear coordinates, say polar coordinates, is a bad idea as we cannot do gradient curves or conical gradients to morph the gradient appropriately. Therefore, varying fill or alpha in non-linear coordinates still gives the previous error.
• The default stat = "align" in geom_area() does not interpolate the fill or alpha at break points, which is then given a single flat fill/alpha.
• In theory, this will also form a gradient between discrete fills if user overrides the default group aesthetic. At the GeomRibbon$draw_group() method, it is no longer possible to discern whether fill was continuous or discrete, so there is no convenient way around this.

[thomasp85]

LGTM