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/flash.go

@@ -0,0 +1,31 @@
+package fx
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type FlashOptions struct {
+	// Cue flashes each time it changes.
+	Cue int
+	// Slot fills the flash; zero means Lit.
+	Slot pixel.Slot
+}
+
+// Flash fills content with a slot one step, then half.
+func Flash(c *ui.Context, o FlashOptions, content func()) *ui.Element {
+	k := pixel.K(c)
+	s := slotOr(o.Slot, pixel.Lit)
+	e := ui.Column(c)
+	// Reduce Motion never flashes.
+	step := k.Shot(c, e, o.Cue, 10, 2)
+	return cover(k, e, func(g pixel.Canvas) {
+		switch step {
+		case 0:
+			g.Rect(0, 0, g.W, g.H, s)
+		case 1:
+			g.Veil(0, 0, g.W, g.H, s, 8)
+		}
+	}, content)
+}