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/starfield.go

@@ -0,0 +1,41 @@
+package fx
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type StarfieldOptions struct {
+	// Width and Height are dots; zeros mean 160 by 96.
+	Width, Height int
+	Label         string
+}
+
+// Starfield scrolls three layers of stars, near ones fastest.
+func Starfield(c *ui.Context, o StarfieldOptions) *ui.Element {
+	k := pixel.K(c)
+	w, h := size("Starfield", o.Width, o.Height, 160, 96)
+	return box(c, k, o.Label, w, h).Draw(func(p *ui.Painter, r ui.Rect) {
+		g := k.Canvas(p, r)
+		g.Clip(0, 0, g.W, g.H, func() { starfield(g, k, g.Step(12)) })
+	})
+}
+
+// starfield moves layer l one dot every 4>>l steps.
+func starfield(g pixel.Canvas, k pixel.Kit, step int) {
+	g.Rect(0, 0, g.W, g.H, dark(k))
+	for l, s := range []pixel.Slot{pixel.Faint, pixel.AccentLit, light(k)} {
+		shift := step / (4 >> l)
+		for i := range g.W * g.H / (120 + 80*l) {
+			x, y := wrap(rnd(i, 20+l)-shift, g.W), rnd(i, 23+l)%g.H
+			switch t := wrap(step+rnd(i, 26), 12); {
+			case l == 2 && t < 2:
+				g.Rect(x-1, y, 3, 1, s)
+				g.Rect(x, y-1, 1, 3, s)
+			case t != 6:
+				g.Dot(x, y, s)
+			}
+		}
+	}
+}