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/mountains.go

@@ -0,0 +1,50 @@
+package fx
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type MountainsOptions struct {
+	// Width and Height are dots; zeros mean 160 by 96.
+	Width, Height int
+	Label         string
+}
+
+// Mountains scroll three ridges: far ones slow, near ones quick.
+func Mountains(c *ui.Context, o MountainsOptions) *ui.Element {
+	k := pixel.K(c)
+	w, h := size("Mountains", 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() { mountains(g, k, g.Step(8), 3) })
+	})
+}
+
+// tri rises from 0 to a, then falls, over p.
+func tri(t, p, a int) int {
+	t, h := wrap(t, p), p/2
+	if t < h {
+		return a * t / h
+	}
+	return a * (p - t) / h
+}
+
+// mountains draws n ridges; ridge l moves every 8>>l steps.
+func mountains(g pixel.Canvas, k pixel.Kit, step, n int) {
+	slots := [3]pixel.Slot{pick(k, pixel.Shade, pixel.Face), pick(k, pixel.Faint, pixel.Back), pick(k, pixel.Edge, pixel.Well)}
+	snowy := k.Palette().Season == pixel.Winter
+	for l := range n {
+		shift := step / (8 >> l)
+		for x := range g.W {
+			u := x + shift
+			h := g.H*(7-2*l)/16 + tri(u+rnd(l, 100), 48+16*l, g.H/5) + tri(2*u+rnd(l, 101), 37, g.H/12) + rnd(u, 102+l)%2
+			g.VLine(x, g.H-h, h, slots[l])
+			// Winter caps the far peaks with snow.
+			if snowy && l == 0 && h > g.H*9/16 {
+				g.VLine(x, g.H-h, 2, light(k))
+			}
+		}
+	}
+}