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/lightning.go

@@ -0,0 +1,65 @@
+package fx
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type LightningOptions struct {
+	// Cue strikes each time it changes.
+	Cue int
+	// Width and Height are dots; zeros mean 96 by 96.
+	Width, Height int
+	Label         string
+}
+
+// strikeSteps run flash, half flash, bolt, bolt, dim bolt.
+const strikeSteps = 5
+
+// Lightning flashes, then forks a bolt to the ground.
+func Lightning(c *ui.Context, o LightningOptions) *ui.Element {
+	k := pixel.K(c)
+	w, h := size("Lightning", o.Width, o.Height, 96, 96)
+	e := box(c, k, o.Label, w, h)
+	step := k.Shot(c, e, o.Cue, 15, strikeSteps)
+	return e.Draw(func(p *ui.Painter, r ui.Rect) {
+		if step == strikeSteps {
+			return
+		}
+		g := k.Canvas(p, r)
+		g.Clip(0, 0, g.W, g.H, func() { strike(g, k, step, o.Cue) })
+	})
+}
+
+// strike draws step of a strike; seed shapes the bolt.
+func strike(g pixel.Canvas, k pixel.Kit, step, seed int) {
+	switch step {
+	case 0:
+		g.Rect(0, 0, g.W, g.H, light(k))
+		return
+	case 1:
+		g.Veil(0, 0, g.W, g.H, light(k), 6)
+	}
+	core, glow := pick(k, pixel.Warn, pixel.Ink), pixel.Info
+	if step == 4 {
+		core = pixel.AccentLit
+	}
+	x, y := g.W/3+rnd(seed, 140)%max(1, g.W/3), 0
+	fork := 1 + rnd(seed, 141)%3
+	for i := 0; y < g.H; i++ {
+		nx, ny := x+rnd(seed+i, 142)%7-3, y+4+rnd(seed+i, 143)%6
+		g.Line(x+1, y, nx+1, ny, glow)
+		g.Line(x, y, nx, ny, core)
+		// One branch forks off and dies out.
+		if i == fork {
+			bx, by, dir := nx, ny, 1-2*(rnd(seed, 144)%2)
+			for j := range 3 {
+				ex, ey := bx+dir*(3+rnd(seed+j, 145)%3), by+3+rnd(seed+j, 146)%4
+				g.Line(bx, by, ex, ey, core)
+				bx, by = ex, ey
+			}
+		}
+		x, y = nx, ny
+	}
+}