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/fx.go

@@ -0,0 +1,95 @@
+// Package fx draws weather, scenes and transitions in stepped frames.
+package fx
+
+import (
+	"fmt"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+// box sizes an effect; a label makes it an image.
+func box(c *ui.Context, k pixel.Kit, label string, w, h int) *ui.Element {
+	e := ui.Box(c).Size(k.D(w), k.D(h))
+	if label != "" {
+		return e.Label(label).Role(ui.RoleImage)
+	}
+	return e.Role(ui.RoleNone)
+}
+
+// size is w×h; zeros take dw×dh.
+func size(name string, w, h, dw, dh int) (int, int) {
+	return pixel.Or(name+" Width", w, dw), pixel.Or(name+" Height", h, dh)
+}
+
+// scale is s, or def for zero; above 4 panics.
+func scale(name string, s, def int) int {
+	s = pixel.Or(name+" Scale", s, def)
+	if s > 4 {
+		panic(fmt.Sprintf("fx: %s Scale %d outside 1–4", name, s))
+	}
+	return s
+}
+
+// rnd is a fixed pseudo-random number for i and salt.
+func rnd(i, salt int) int {
+	x := uint64(i)*0x9E3779B97F4A7C15 + uint64(salt)*0xD1B54A32D192ED03
+	x = (x ^ x>>30) * 0xBF58476D1CE4E5B9
+	x = (x ^ x>>27) * 0x94D049BB133111EB
+	return int((x ^ x>>31) >> 1)
+}
+
+// wrap is v modulo n, never negative.
+func wrap(v, n int) int { return (v%n + n) % n }
+
+// sway swings 0 to a, back through -a, over p.
+func sway(t, p, a int) int {
+	t = wrap(t, p)
+	q := p / 4
+	switch {
+	case t < q:
+		return a * t / q
+	case t < 3*q:
+		return a - 2*a*(t-q)/(2*q)
+	default:
+		return -a + a*(t-3*q)/q
+	}
+}
+
+// pick is day under day palettes, night under night ones.
+func pick(k pixel.Kit, day, night pixel.Slot) pixel.Slot {
+	if k.Palette().Night {
+		return night
+	}
+	return day
+}
+
+// light is the palette's brightest slot.
+func light(k pixel.Kit) pixel.Slot { return pick(k, pixel.Lit, pixel.Ink) }
+
+// dark is the palette's darkest slot.
+func dark(k pixel.Kit) pixel.Slot { return pick(k, pixel.Ink, pixel.Well) }
+
+// slotOr is s, or def for the zero slot.
+func slotOr(s, def pixel.Slot) pixel.Slot {
+	if s == pixel.Ink {
+		return def
+	}
+	return s
+}
+
+// checkHour panics outside 0–23.
+func checkHour(name string, hour int) {
+	if hour < 0 || hour > 23 {
+		panic(fmt.Sprintf("fx: %s hour %d outside 0–23", name, hour))
+	}
+}
+
+// cover draws over e's content, clipped to e.
+func cover(k pixel.Kit, e *ui.Element, draw func(g pixel.Canvas), content func()) *ui.Element {
+	return e.DrawOver(func(p *ui.Painter, r ui.Rect) {
+		g := k.Canvas(p, r)
+		g.Clip(0, 0, g.W, g.H, func() { draw(g) })
+	}).Children(content)
+}