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.

fx/sparkle.go

@@ -0,0 +1,48 @@
+package fx
+
+import (
+	"math"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type SparkleOptions struct {
+	// Size is the square's side in dots; zero means 24.
+	Size int
+	// Slot inks the stars; zero means Warn.
+	Slot  pixel.Slot
+	Label string
+}
+
+// glints are a star's frames by step; -1 is dark.
+var glints = [...]int{0, 1, 2, 1, 0, -1, -1, -1}
+
+// Sparkle twinkles four stars around its middle, in turn.
+func Sparkle(c *ui.Context, o SparkleOptions) *ui.Element {
+	k := pixel.K(c)
+	n := pixel.Or("Sparkle Size", o.Size, 24)
+	s := slotOr(o.Slot, pixel.Warn)
+	return box(c, k, o.Label, n, n).Draw(func(p *ui.Painter, r ui.Rect) {
+		g := k.Canvas(p, r)
+		g.Clip(0, 0, g.W, g.H, func() { sparkle(g, s, g.Step(10)) })
+	})
+}
+
+// sparkle moves each star to a new spot every cycle.
+func sparkle(g pixel.Canvas, s pixel.Slot, step int) {
+	for i := range 4 {
+		t := step + 2*i
+		f := glints[wrap(t, len(glints))]
+		if f < 0 {
+			continue
+		}
+		seed := t/len(glints)*4 + i
+		a := float64(i)*math.Pi/2 + float64(rnd(seed, 170)%60-30)*math.Pi/180
+		d := float64(g.W) * float64(20+rnd(seed, 171)%20) / 100
+		x := g.W/2 + int(math.Round(d*math.Cos(a))) - 2
+		y := g.H/2 + int(math.Round(d*math.Sin(a))) - 2
+		g.Sprite(x, y, twinkle, f, s, 1)
+	}
+}