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/av-elier/go-decimal-to-rational/frac.go

@@ -0,0 +1,44 @@
+package dectofrac
+
+import (
+	"math"
+	"math/big"
+)
+
+// MaxIterations is some sane limit of iterations for precision mode
+const MaxIterations = 5000
+
+// NewRatI returns rational from decimal
+// using `iterations` number of iterations in Continued Fraction algorythm
+func NewRatI(val float64, iterations int64) *big.Rat {
+	return NewRat(val, iterations, 0)
+}
+
+// NewRatP returns rational from decimal
+// by going as mush iterations, until next fraction is less than `stepPrecision`
+func NewRatP(val float64, stepPrecision float64) *big.Rat {
+	return NewRat(val, MaxIterations, stepPrecision)
+}
+
+func NewRat(val float64, iterations int64, stepPrecision float64) *big.Rat {
+	a0 := int64(math.Floor(val))
+	x0 := val - float64(a0)
+	rat := cf(x0, 1, iterations, stepPrecision)
+	return rat.Add(rat, new(big.Rat).SetInt64(a0))
+}
+
+func cf(xi float64, i int64, limit int64, stepPrecision float64) *big.Rat {
+	if i >= limit || xi <= stepPrecision {
+		return big.NewRat(0, 1)
+	}
+
+	inverted := 1 / xi
+	aj := int64(math.Floor(inverted))
+	xj := inverted - float64(aj)
+	ratAJ := new(big.Rat).SetInt64(aj)
+	ratNext := cf(xj, i+1, limit, stepPrecision)
+	res := ratAJ.Add(ratAJ, ratNext)
+	res = res.Inv(res)
+
+	return res
+}