Support Telegram animated stickers (tgs) format (#1173)

This is half a fix for #874

This patch introduces a new config flag:
- MediaConvertTgs

These need to be treated independently from the existing
MediaConvertWebPToPNG flag because Tgs→WebP results in an
*animated* WebP, and the WebP→PNG converter can't handle
animated WebP files yet.

Furthermore, some platforms (like discord) don't even support
animated WebP files, so the user may want to fall back to
static PNGs (not APNGs).

The final reason why this is only half a fix is that this
introduces an external dependency, namely lottie, to be
installed like this:

$ pip3 install lottie cairosvg

This patch works by writing the tgs to a temporary file in /tmp,
calling lottie to convert it (this conversion may take several seconds!),
and then deleting the temporary file.
The temporary file is absolutely necessary, as lottie refuses to
work on non-seekable files.
If anyone comes up with a reasonable use case where /tmp is
unavailable, I can add yet another config option for that, if desired.

Telegram will bail out if the option is configured but lottie isn't found.

bridge/helper/helper.go

@@ -5,7 +5,10 @@ import (
 	"fmt"
 	"image/png"
 	"io"
+	"io/ioutil"
 	"net/http"
+	"os"
+	"os/exec"
 	"regexp"
 	"strings"
 	"time"
@@ -192,7 +195,7 @@ func ParseMarkdown(input string) string {
 	return res
 }
 
-// ConvertWebPToPNG convert input data (which should be WebP format to PNG format)
+// ConvertWebPToPNG converts input data (which should be WebP format) to PNG format
 func ConvertWebPToPNG(data *[]byte) error {
 	r := bytes.NewReader(*data)
 	m, err := webp.Decode(r)
@@ -207,3 +210,49 @@ func ConvertWebPToPNG(data *[]byte) error {
 	*data = w.Bytes()
 	return nil
 }
+
+// CanConvertTgsToX Checks whether the external command necessary for ConvertTgsToX works.
+func CanConvertTgsToX() error {
+	// We depend on the fact that `lottie_convert.py --help` has exit status 0.
+	// Hyrum's Law predicted this, and Murphy's Law predicts that this will break eventually.
+	// However, there is no alternative like `lottie_convert.py --is-properly-installed`
+	cmd := exec.Command("lottie_convert.py", "--help")
+	return cmd.Run()
+}
+
+// ConvertTgsToWebP convert input data (which should be tgs format) to WebP format
+// This relies on an external command, which is ugly, but works.
+func ConvertTgsToX(data *[]byte, outputFormat string, logger *logrus.Entry) error {
+	// lottie can't handle input from a pipe, so write to a temporary file:
+	tmpFile, err := ioutil.TempFile(os.TempDir(), "matterbridge-lottie-*.tgs")
+	if err != nil {
+		return err
+	}
+	tmpFileName := tmpFile.Name()
+	defer func() {
+		if removeErr := os.Remove(tmpFileName); removeErr != nil {
+			logger.Errorf("Could not delete temporary file %s: %v", tmpFileName, removeErr)
+		}
+	}()
+
+	if _, writeErr := tmpFile.Write(*data); writeErr != nil {
+		return writeErr
+	}
+	// Must close before calling lottie to avoid data races:
+	if closeErr := tmpFile.Close(); closeErr != nil {
+		return closeErr
+	}
+
+	// Call lottie to transform:
+	cmd := exec.Command("lottie_convert.py", "--input-format", "lottie", "--output-format", outputFormat, tmpFileName, "/dev/stdout")
+	cmd.Stderr = nil
+	// NB: lottie writes progress into to stderr in all cases.
+	stdout, stderr := cmd.Output()
+	if stderr != nil {
+		// 'stderr' already contains some parts of Stderr, because it was set to 'nil'.
+		return stderr
+	}
+
+	*data = stdout
+	return nil
+}