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/dissolve.go

@@ -0,0 +1,39 @@
+package fx
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type DissolveOptions struct {
+	// Slot replaces the dots; zero means the darkest slot.
+	Slot pixel.Slot
+}
+
+// Dissolve switches content's dots to a slot in shuffled order.
+func Dissolve(c *ui.Context, covered bool, o DissolveOptions, content func()) *ui.Element {
+	k := pixel.K(c)
+	s := slotOr(o.Slot, dark(k))
+	e := ui.Column(c)
+	step := k.Toward(c, e, covered, 12, 8)
+	return cover(k, e, func(g pixel.Canvas) { dissolve(g, s, step) }, content)
+}
+
+// dissolve covers dots whose fixed rank, 0–7, is below step.
+func dissolve(g pixel.Canvas, s pixel.Slot, step int) {
+	on := func(x, y int) bool { return rnd(x+y<<16, 150)%8 < step }
+	for y := range g.H {
+		for x := 0; x < g.W; x++ {
+			if !on(x, y) {
+				continue
+			}
+			end := x + 1
+			for end < g.W && on(end, y) {
+				end++
+			}
+			g.HLine(x, y, end-x, s)
+			x = end
+		}
+	}
+}