Fix index out of bounds in simplify_gt.gtable_patchwork()

[frvirard]

Summary

simplify_gt.gtable_patchwork() computes the collapsed height of a nested
patchwork from gt$widths[panel_pos$t:panel_pos$b] instead of
gt$heights[panel_pos$t:panel_pos$b]:

if (all(is_abs_unit(gt$heights[panel_pos$t:panel_pos$b]))) {
  new_height <- sum(convertHeight(gt$widths[panel_pos$t:panel_pos$b], 'mm'))
  #                                ^^^^^^^ should be gt$heights
} else {
  new_height <- unit(1, 'null')
}

gt$widths only has one entry per column, so as soon as a nested block's
row range (panel_pos$t:panel_pos$b) exceeds the table's column count -
which happens whenever every height in the nested layout is an absolute
unit (unit(x, "cm")/"pt"/"in", i.e. is_abs_unit() is TRUE for all
of them) - the index goes out of bounds and printing/building the
patchwork fails with:

Error: index out of bounds ("unit" subsetting)

Using a context-relative unit (e.g. "line") for at least one height
avoids the bug, since is_abs_unit() then returns FALSE and the safe
else branch is taken instead - that's how I traced it back to this line.
Introduced in 556874d ("General solution to inheriting dimensions from
plot/table").

Reproducible example

library(ggplot2)
library(patchwork)
library(grid)

make_block <- function(n_lines) {
  title <- textGrob(paste(rep("label line", n_lines), collapse = "\n"))
  plot  <- ggplot(mtcars, aes(mpg, wt)) + geom_point()
  wrap_elements(title) + plot +
    plot_layout(ncol = 1, heights = unit.c(unit(n_lines, "cm"), unit(5, "cm")))
}

combined <- wrap_plots(list(make_block(1), make_block(3)), ncol = 1)
ggsave("reprex.pdf", combined, width = 5, height = 8)
#> Error: index out of bounds ("unit" subsetting)

Changes

• R/plot_patchwork.R: one-line fix, gt$widths → gt$heights in the
  new_height branch.
• tests/testthat/test-layout.R: regression test reproducing the crash
  above and asserting patchworkGrob() no longer errors on it.
• NEWS.md: changelog entry.

Test plan

• Confirmed the added test fails against unpatched patchwork 1.3.2
  and passes after the fix (verified locally, both via a direct
  patchworkGrob() call and via testthat::test_file()).
• Ran tests/testthat/test-layout.R with pkgload::load_all(); no
  failures (pre-existing vdiffr snapshot tests skip in this
  environment, unrelated to this change).