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/iriswipe.go

@@ -0,0 +1,42 @@
+package fx
+
+import (
+	"math"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type IrisWipeOptions struct {
+	// Slot fills outside the iris; zero means the darkest slot.
+	Slot pixel.Slot
+}
+
+// IrisWipe closes a circle on content's middle, or opens it.
+func IrisWipe(c *ui.Context, closed bool, o IrisWipeOptions, content func()) *ui.Element {
+	k := pixel.K(c)
+	s := slotOr(o.Slot, dark(k))
+	e := ui.Column(c)
+	step := k.Toward(c, e, closed, 12, 8)
+	return cover(k, e, func(g pixel.Canvas) { iris(g, s, step) }, content)
+}
+
+// iris fills outside a circle, gone by step 8.
+func iris(g pixel.Canvas, s pixel.Slot, step int) {
+	if step == 0 {
+		return
+	}
+	cx, cy := g.W/2, g.H/2
+	r := int(math.Ceil(math.Hypot(float64(g.W)/2, float64(g.H)/2))) * (8 - step) / 8
+	for y := range g.H {
+		dy := y - cy
+		if dy*dy >= r*r {
+			g.HLine(0, y, g.W, s)
+			continue
+		}
+		dx := int(math.Sqrt(float64(r*r - dy*dy)))
+		g.HLine(0, y, cx-dx, s)
+		g.HLine(cx+dx, y, g.W-cx-dx, s)
+	}
+}