fix #17

[ntjess]

Fix #17 by providing a more generalized cover function. Works with several content types (not just text) and on several background colors instead of assuming white. Sample usage:

#import "polylux.typ": *

#set page(width: 2in, height: 2.2in, background: rect(width: 100%, height: 100%))

#let slide = polylux-slide

#let img = image("assets/logo.png", height: 0.5in)


#slide[
  = Hello, world

  #uncover(2, mode: "transparent", img)

  #set text(fill: white)
  #let bg(fill, h: 45%) = place(rect(width: 100%, height: h, fill: fill))

  // Typst `green` is not true green
  #add-hider-mode("green", cover-with-rect.with(fill: rgb(46, 204, 64, 90%)))
  // Use "transparent" on black background as default when no
  // mode is specified
  #add-hider-mode("default", logic.cover-with-black-rect)
  #bg(black)
  #uncover(2, [Some text])
  #side-by-side[
    #uncover(2, img)
  ][
    // Let the user specify their own modes when operating on nonstandard background
    // colors
    #uncover(2, mode: "green", box(img, fill: green, outset: 1em))
  ]
]

Produces:

[ntjess]

For some reason, GitHub isn't updating this PR in synch with changes to my main branch. Closing this PR so I can reopen another that's up-to-date.