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/clouds.go

@@ -0,0 +1,36 @@
+package fx
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type CloudsOptions struct {
+	// Width and Height are dots; zeros mean 160 by 96.
+	Width, Height int
+	Label         string
+}
+
+// Clouds drift by: far ones slowly, near ones faster.
+func Clouds(c *ui.Context, o CloudsOptions) *ui.Element {
+	k := pixel.K(c)
+	w, h := size("Clouds", 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() { clouds(g, g.Step(8)) })
+	})
+}
+
+// clouds drifts two layers across the top third of g.
+func clouds(g pixel.Canvas, step int) {
+	for layer, sp := range []*pixel.Sprite{smallCloud, bigCloud} {
+		// Far clouds shift a dot each four steps; near, two.
+		shift, span := step/(4>>layer), g.W+sp.W
+		for i := range max(1, g.W/(120-20*layer)) {
+			x := wrap(rnd(i, 10+layer)-shift, span) - sp.W
+			y := rnd(i, 12+layer)%max(1, g.H/3) + 4*layer
+			g.Sprite(x, y, sp, 0, pixel.Ink, 1)
+		}
+	}
+}