Add fx components 291-320 and the home scene; Kit.Shot, Kit.Toward

Palette cycling, sky, sun and moon, clouds, starfield, rain, snow,
petals, leaves, fireflies, waterfall, ocean, campfire, lava, aurora,
lightning, mountains and a season scene; scanlines, vignette, screen
shake, flash, dither fades, iris wipes, dissolves, mosaics, glitches,
sparkles, explosions and confetti. Home opens on a live scene in the
palette's season and a strip of all eight palettes.

pixel adds Canvas.Cycled, Sprite.At, SpriteOf, Kit.Toward and
Kit.Shot, which spends a cue under Reduce Motion so it never replays;
display exports Reduce. Gallery grids and the sidebar are one Tab
stop each.

game/letterbox.go

@@ -1,8 +1,6 @@
 package game
 
 import (
-	"time"
-
 	"github.com/egoist/mygo/ui"
 
 	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/layout"
@@ -14,38 +12,13 @@ type LetterboxOptions struct {
 	Bar int
 }
 
-type barsKey struct{}
-
-// bars is the last on, its change time and step.
-type bars struct {
-	on   bool
-	at   time.Time
-	from int
-}
-
 // Letterbox slides cutscene bars over content in four steps.
 func Letterbox(c *ui.Context, on bool, o LetterboxOptions, content func()) *ui.Element {
 	k := pixel.K(c)
 	bar := pixel.Or("Letterbox Bar", o.Bar, 16)
 	var cover *ui.Element
 	e := layout.Layers(c, content, func() { cover = ui.Box(c).Fill().PassThrough() })
-	// First shown, the bars are already where on puts them.
-	s := ui.Local(e, barsKey{}, func() bars { return bars{on: on, from: 4 * boolInt(on)} })
-	// shown is the step, 0–4; turns start where it is.
-	shown := func() int {
-		if k.Reduce() {
-			return 4 * boolInt(s.on)
-		}
-		if n := k.Since(c, s.at, 8, 4); s.on {
-			return min(4, s.from+n)
-		} else {
-			return max(0, s.from-n)
-		}
-	}
-	if on != s.on {
-		s.from, s.on, s.at = shown(), on, k.Now()
-	}
-	h := bar * shown() / 4
+	h := bar * k.Toward(c, e, on, 8, 4) / 4
 	cover.Draw(func(p *ui.Painter, r ui.Rect) {
 		g := k.Canvas(p, r)
 		g.Rect(0, 0, g.W, h, pixel.Ink)
@@ -53,10 +26,3 @@ func Letterbox(c *ui.Context, on bool, o LetterboxOptions, content func()) *ui.E
 	})
 	return e
 }
-
-func boolInt(b bool) int {
-	if b {
-		return 1
-	}
-	return 0
-}