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/vignette.go

@@ -0,0 +1,40 @@
+package fx
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type VignetteOptions struct {
+	// Slot darkens the edges; zero means the darkest slot.
+	Slot pixel.Slot
+}
+
+// Vignette dithers a slot inward from edges; corners go solid.
+func Vignette(c *ui.Context, o VignetteOptions, content func()) *ui.Element {
+	k := pixel.K(c)
+	s := slotOr(o.Slot, dark(k))
+	return cover(k, ui.Column(c), func(g pixel.Canvas) { vignette(g, s) }, content)
+}
+
+// vignette's level grows with the elliptic distance out.
+func vignette(g pixel.Canvas, s pixel.Slot) {
+	level := func(x, y int) int {
+		ex := float64(2*x+1-g.W) / float64(g.W)
+		ey := float64(2*y+1-g.H) / float64(g.H)
+		return min(16, max(0, int((ex*ex+ey*ey-0.6)*16/1.2)))
+	}
+	for y := range g.H {
+		for x := 0; x < g.W; {
+			l, end := level(x, y), x+1
+			for end < g.W && level(end, y) == l {
+				end++
+			}
+			if l > 0 {
+				g.Veil(x, y, end-x, 1, s, l)
+			}
+			x = end
+		}
+	}
+}