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/sunmoon.go

@@ -0,0 +1,54 @@
+package fx
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type SunMoonOptions struct {
+	// Width and Height are dots; zeros mean 160 by 96.
+	Width, Height int
+	Label         string
+}
+
+// SunMoon arcs the sun by day, the moon by night.
+func SunMoon(c *ui.Context, hour int, o SunMoonOptions) *ui.Element {
+	checkHour("SunMoon", hour)
+	k := pixel.K(c)
+	w, h := size("SunMoon", 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() { sunMoon(g, k, hour, g.Step(4)) })
+	})
+}
+
+// arc centers the body: low at the ends, high midway.
+func arc(w, h, hour int) (int, int) {
+	t := wrap(hour-6, 12)
+	horizon, top := h*3/4, 10
+	x := 10 + (w-20)*(2*t+1)/24
+	return x, horizon - (horizon-top)*(2*t+1)*(23-2*t)/143
+}
+
+// sunMoon draws a sun with turning rays, or cratered moon.
+func sunMoon(g pixel.Canvas, k pixel.Kit, hour, step int) {
+	x, y := arc(g.W, g.H, hour)
+	if hour < 6 || hour >= 18 {
+		g.Disc(x, y, 5, light(k))
+		g.Dot(x-2, y-1, pixel.Faint)
+		g.Rect(x+1, y+1, 2, 1, pixel.Faint)
+		g.Dot(x+2, y-3, pixel.Faint)
+		return
+	}
+	// Rays turn an eighth each step: straight, then slanted.
+	for i := range 4 {
+		dx, dy := [4]int{1, 0, -1, 0}[i], [4]int{0, 1, 0, -1}[i]
+		if step%2 == 1 {
+			dx, dy = [4]int{1, 1, -1, -1}[i], [4]int{1, -1, 1, -1}[i]
+		}
+		g.Line(x+7*dx, y+7*dy, x+8*dx, y+8*dy, pixel.Warn)
+	}
+	g.Disc(x, y, 5, pixel.Warn)
+	g.Rect(x-2, y-2, 2, 2, light(k))
+}