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/glitch.go

@@ -0,0 +1,64 @@
+package fx
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type GlitchOptions struct {
+	// Cue glitches the picture each time it changes.
+	Cue int
+	// Slot inks the sprite's # dots.
+	Slot  pixel.Slot
+	Scale int
+	Label string
+}
+
+const glitchSteps = 6
+
+// Glitch shifts row bands and swaps two slots, briefly.
+func Glitch(c *ui.Context, sp *pixel.Sprite, o GlitchOptions) *ui.Element {
+	k := pixel.K(c)
+	sc := scale("Glitch", o.Scale, 1)
+	// Three dots either side hold the rows shifted out.
+	e := box(c, k, o.Label, (sp.W+6)*sc, sp.H*sc)
+	step := k.Shot(c, e, o.Cue, 15, glitchSteps)
+	used := slotsOf(sp, o.Slot)
+	return e.Draw(func(p *ui.Painter, r ui.Rect) {
+		g := k.Canvas(p, r)
+		if step == glitchSteps {
+			g.Sprite(3*sc, 0, sp, 0, o.Slot, sc)
+			return
+		}
+		seed := o.Cue*glitchSteps + step
+		if len(used) > 1 {
+			a := rnd(seed, 160) % len(used)
+			b := (a + 1 + rnd(seed, 161)%(len(used)-1)) % len(used)
+			g = g.Cycled([]pixel.Slot{used[a], used[b]}, 1)
+		}
+		for y := 0; y < sp.H; {
+			h, dx := 1+rnd(seed+y, 162)%4, 0
+			if rnd(seed+y, 163)%3 == 0 {
+				dx = rnd(seed+y, 164)%7 - 3
+			}
+			g.Clip(0, y*sc, g.W, h*sc, func() { g.Sprite((3+dx)*sc, 0, sp, 0, o.Slot, sc) })
+			y += h
+		}
+	})
+}
+
+// slotsOf lists the slots sp's first frame uses.
+func slotsOf(sp *pixel.Sprite, ink pixel.Slot) []pixel.Slot {
+	var seen [pixel.Slots]bool
+	var out []pixel.Slot
+	for y := range sp.H {
+		for x := range sp.W {
+			if s, ok := sp.At(0, x, y, ink); ok && !seen[s] {
+				seen[s] = true
+				out = append(out, s)
+			}
+		}
+	}
+	return out
+}