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/README.md

@@ -0,0 +1,54 @@
+# go-decimal-to-rational
+
+[![Build Status](https://travis-ci.org/av-elier/go-decimal-to-rational.svg?branch=master)](https://travis-ci.org/av-elier/go-decimal-to-rational)
+
+Go library to convert decimal (float64) to rational fraction with required precision
+
+Relies on [Continued Fraction](http://mathworld.wolfram.com/ContinuedFraction.html) algorythm.
+
+It's sometimes more appropriate than default big.Rat SetString, because
+you can get `2/3` from `0.6666` by specifiing required precision. In big.Rat SetString
+you can only get `3333/50000`, and have no way to manipulate than (as of go 1.11).
+
+# Example
+```go
+func ExampleNewRatP() {
+	fmt.Println(NewRatP(0.6666, 0.01).String())
+	fmt.Println(NewRatP(0.981, 0.001).String())
+	fmt.Println(NewRatP(0.75, 0.01).String())
+	// Output:
+	// 2/3
+	// 981/1000
+	// 3/4
+}
+```
+```go
+func ExampleNewRatI() {
+	fmt.Println(NewRatI(0.6667, 3).String())
+	fmt.Println(NewRatI(0.6667, 4).String())
+	// Output:
+	// 2/3
+	// 6667/10000
+}
+```
+
+# Docs
+```
+import dectofrac "github.com/av-elier/go-decimal-to-rational"
+```
+
+#### func NewRatI
+
+```go
+func NewRatI(val float64, iterations int64) *big.Rat
+```
+NewRatI returns rational from decimal using `iterations` number of
+iterations in Continued Fraction algorythm
+
+#### func NewRatP
+
+```go
+func NewRatP(val float64, stepPrecision float64) *big.Rat
+```
+NewRatP returns rational from decimal by going as mush iterations, until
+next fraction is less than `stepPrecision`