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/scanlines.go

@@ -0,0 +1,23 @@
+package fx
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type ScanlinesOptions struct {
+	// Slot shades the rows; zero means Shade.
+	Slot pixel.Slot
+}
+
+// Scanlines shades every other row of content at half strength.
+func Scanlines(c *ui.Context, o ScanlinesOptions, content func()) *ui.Element {
+	k := pixel.K(c)
+	s := slotOr(o.Slot, pixel.Shade)
+	return cover(k, ui.Column(c), func(g pixel.Canvas) {
+		for y := 1; y < g.H; y += 2 {
+			g.Checker(0, y, g.W, 1, s)
+		}
+	}, content)
+}