Avoid bounded density discontinuity

[teunbrand]

This PR aims to fix #5641.

Briefly, the way bounded densities were estimated may result in discontinuities due to incomplete estimation of the tails.
As densities are 'mirrored' across finite bounds, in this PR we ensure that the 'mirror image' is the same size as the original by multiplying the to/from range in density estimation. We also increase n to not lose any detail in the bounded density.

The reprex from #5641 with this PR now shows a flatter line in the uniform density:

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

x <- ppoints(100)
p <- ggplot(data.frame(x), aes(x)) + 
  geom_density(bounds = c(0,1))
p

p + coord_cartesian(ylim = c(.99, 1.01))

^{Created on 2024-01-15 with reprex v2.0.2}

[teunbrand]

@mjskay Could I ask you if you think this implementation is correct?

[mjskay]

Yeah this looks right to me!

[thomasp85]

LGTM