Skip alignment/stacking when possible

[teunbrand]

This PR aims to fix #5788.

Briefly, when there is only 1 group with unique x variables, stat_align() + position_stack() can be slow. By skipping alignment when there is only 1 group, we go down from ~2 seconds to ~700 milliseconds for 10e4 observations. By skipping stacking when all x positions are unique, we go from ~700 milliseconds to ~70 milliseconds. All in all, I'd say that 1 group, all unique x is common enough to make the skipping worth it.

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

dat <- data.frame(
  x = 1:1e4,
  y = rnorm(1e4) + 5
)

area   <- ggplot(dat) + geom_area(aes(x, y))
ribbon <- ggplot(dat) + geom_ribbon(aes(x, ymin = 0, ymax = y))

ragg::agg_png(tempfile(fileext = ".png"))

res <- bench::mark(
  area   = print(area),
  ribbon = print(ribbon),
  check  = FALSE,
  min_iterations = 5
)
print(res)
#> # A tibble: 2 × 13
#>   expression      min median `itr/sec` mem_alloc `gc/sec` n_itr  n_gc total_time
#>   <bch:expr> <bch:tm> <bch:>     <dbl> <bch:byt>    <dbl> <int> <dbl>   <bch:tm>
#> 1 area         76.5ms 76.8ms      12.1    28.3MB     9.05     4     3      331ms
#> 2 ribbon       74.1ms 75.1ms      13.3    23.6MB     9.94     4     3      302ms
#> # ℹ 4 more variables: result <list>, memory <list>, time <list>, gc <list>

^{Created on 2024-03-21 with reprex v2.1.0}