Convert .tgs with go libraries (and cgo) (telegram) (#1569)

This commit adds support for go/cgo tgs conversion when building with the -tags `cgo`
The default binaries are still "pure" go and uses the old way of converting.

* Move lottie_convert.py conversion code to its own file

* Add optional libtgsconverter

* Update vendor

* Apply suggestions from code review

* Update bridge/helper/libtgsconverter.go

Co-authored-by: Wim <wim@42.be>

vendor/github.com/Benau/tgsconverter/libtgsconverter/quantize_bucket.go

@@ -0,0 +1,119 @@
+package libtgsconverter
+
+import "image/color"
+
+type colorAxis uint8
+
+// Color axis constants
+const (
+	red colorAxis = iota
+	green
+	blue
+)
+
+type colorPriority struct {
+	p uint32
+	color.RGBA
+}
+
+func (c colorPriority) axis(span colorAxis) uint8 {
+	switch span {
+	case red:
+		return c.R
+	case green:
+		return c.G
+	default:
+		return c.B
+	}
+}
+
+type colorBucket []colorPriority
+
+func (cb colorBucket) partition() (colorBucket, colorBucket) {
+	mean, span := cb.span()
+	left, right := 0, len(cb)-1
+	for left < right {
+		cb[left], cb[right] = cb[right], cb[left]
+		for cb[left].axis(span) < mean && left < right {
+			left++
+		}
+		for cb[right].axis(span) >= mean && left < right {
+			right--
+		}
+	}
+	if left == 0 {
+		return cb[:1], cb[1:]
+	}
+	if left == len(cb)-1 {
+		return cb[:len(cb)-1], cb[len(cb)-1:]
+	}
+	return cb[:left], cb[left:]
+}
+
+func (cb colorBucket) mean() color.RGBA {
+	var r, g, b uint64
+	var p uint64
+	for _, c := range cb {
+		p += uint64(c.p)
+		r += uint64(c.R) * uint64(c.p)
+		g += uint64(c.G) * uint64(c.p)
+		b += uint64(c.B) * uint64(c.p)
+	}
+	return color.RGBA{uint8(r / p), uint8(g / p), uint8(b / p), 255}
+}
+
+type constraint struct {
+	min  uint8
+	max  uint8
+	vals [256]uint64
+}
+
+func (c *constraint) update(index uint8, p uint32) {
+	if index < c.min {
+		c.min = index
+	}
+	if index > c.max {
+		c.max = index
+	}
+	c.vals[index] += uint64(p)
+}
+
+func (c *constraint) span() uint8 {
+	return c.max - c.min
+}
+
+func (cb colorBucket) span() (uint8, colorAxis) {
+	var R, G, B constraint
+	R.min = 255
+	G.min = 255
+	B.min = 255
+	var p uint64
+	for _, c := range cb {
+		R.update(c.R, c.p)
+		G.update(c.G, c.p)
+		B.update(c.B, c.p)
+		p += uint64(c.p)
+	}
+	var toCount *constraint
+	var span colorAxis
+	if R.span() > G.span() && R.span() > B.span() {
+		span = red
+		toCount = &R
+	} else if G.span() > B.span() {
+		span = green
+		toCount = &G
+	} else {
+		span = blue
+		toCount = &B
+	}
+	var counted uint64
+	var i int
+	var c uint64
+	for i, c = range toCount.vals {
+		if counted > p/2 || counted+c == p {
+			break
+		}
+		counted += c
+	}
+	return uint8(i), span
+}