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/aurora.go

@@ -0,0 +1,35 @@
+package fx
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type AuroraOptions struct {
+	// Width and Height are dots; zeros mean 160 by 96.
+	Width, Height int
+	Label         string
+}
+
+// Aurora hangs curtains whose slots slide sideways.
+func Aurora(c *ui.Context, o AuroraOptions) *ui.Element {
+	k := pixel.K(c)
+	w, h := size("Aurora", 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() { aurora(g, g.Step(8)) })
+	})
+}
+
+// aurora's columns ripple on top and fade at the hem.
+func aurora(g pixel.Canvas, step int) {
+	band := pixel.Cycle([]pixel.Slot{pixel.Good, pixel.Info, pixel.Magic, pixel.Good, pixel.AccentLit}, step)
+	for x := range g.W {
+		top := g.H/8 + tri(x/2+step, 64, g.H/6) + tri(x+2*step, 36, 3)
+		long := g.H/4 + tri(3*x/2-step, 50, g.H/6)
+		s := band[x/4%len(band)]
+		g.VLine(x, top, long*2/3, s)
+		g.Veil(x, top+long*2/3, 1, long/3, s, 6)
+	}
+}