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/sky.go

@@ -0,0 +1,63 @@
+package fx
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type SkyOptions struct {
+	// Width and Height are dots; zeros mean 160 by 96.
+	Width, Height int
+	Label         string
+}
+
+// Sky dithers down the palette's sky slots; night shows stars.
+func Sky(c *ui.Context, o SkyOptions) *ui.Element {
+	k := pixel.K(c)
+	w, h := size("Sky", 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() { sky(g, k, g.Step(4)) })
+	})
+}
+
+// sky fills g in bands cut at height percents.
+func sky(g pixel.Canvas, k pixel.Kit, step int) {
+	bands, cuts := []pixel.Slot{pixel.Info, pixel.AccentLit, pixel.Lit}, []int{0, 55, 85, 100}
+	if k.Palette().Night {
+		bands, cuts = []pixel.Slot{pixel.Well, pixel.Back, pixel.Face, pixel.Lit}, []int{0, 35, 60, 82, 100}
+	}
+	for i, s := range bands {
+		y0, y1 := g.H*cuts[i]/100, g.H*cuts[i+1]/100
+		g.Rect(0, y0, g.W, y1-y0, s)
+	}
+	// Seams dither five rows from one band into the next.
+	for i := 1; i < len(bands); i++ {
+		y := g.H * cuts[i] / 100
+		for j := range 5 {
+			g.Dither(0, y-2+j, g.W, 1, bands[i-1], bands[i], 2+3*j)
+		}
+	}
+	if k.Palette().Night {
+		stars(g, k, step)
+	}
+}
+
+// stars twinkle over the top two thirds.
+func stars(g pixel.Canvas, k pixel.Kit, step int) {
+	tones := []pixel.Slot{light(k), light(k), pixel.AccentLit, light(k), pixel.Warn}
+	for i := range g.W * g.H / 90 {
+		x, y := rnd(i, 1)%g.W, rnd(i, 2)%max(1, g.H*2/3)
+		s, t := tones[i%len(tones)], wrap(step+rnd(i, 3), 8)
+		switch {
+		case t == 0:
+			g.Dot(x, y, pixel.Faint)
+		case i%9 == 0 && t < 3:
+			g.Rect(x-1, y, 3, 1, s)
+			g.Rect(x, y-1, 1, 3, s)
+		default:
+			g.Dot(x, y, s)
+		}
+	}
+}