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/rain.go

@@ -0,0 +1,44 @@
+package fx
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type RainOptions struct {
+	// Width and Height are dots; zeros mean 160 by 96.
+	Width, Height int
+	Label         string
+}
+
+// Rain slants streaks down; each splashes as it lands.
+func Rain(c *ui.Context, o RainOptions) *ui.Element {
+	k := pixel.K(c)
+	w, h := size("Rain", 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() { rain(g, k, g.Step(12)) })
+	})
+}
+
+// rain drops fall three rows, a dot left, each step.
+func rain(g pixel.Canvas, k pixel.Kit, step int) {
+	s := pick(k, pixel.Faint, pixel.Info)
+	fall := (g.H+4)/3 + 1
+	for i := range g.W * g.H / 160 {
+		p := wrap(step+rnd(i, 30), fall+3)
+		x := wrap(rnd(i, 31)-p, g.W)
+		switch {
+		case p < fall:
+			y := 3*p - 4
+			g.Line(x+1, y, x, y+3, s)
+		case p == fall:
+			g.Dot(x-1, g.H-2, s)
+			g.Dot(x+1, g.H-2, s)
+		case p == fall+1:
+			g.Dot(x-2, g.H-3, s)
+			g.Dot(x+2, g.H-3, s)
+		}
+	}
+}