Updating mlt to 6.2.0

(cherry picked from commit 93d8ab8007102e0e4d7f23cf25bb353d1cc5bced)

I checked with kdenlive people, and they say that we should always use the
latest mlt possible; that it should not be any problem, and provide only
improvements.

pkgs/development/libraries/mlt/qt-5.nix

···
 
 stdenv.mkDerivation rec {
   name = "mlt-${version}";
-  version = "0.9.8";
+  version = "6.2.0";
 
   src = fetchurl {
     url = "https://github.com/mltframework/mlt/archive/v${version}.tar.gz";
-    sha256 = "0x0hsb05i7g70dh3jll41qlvcylailfgsr0y1dpx7hyigynxc50j";
+    sha256 = "1zwzfgxrcbwkxnkiwv0a1rzxdnnaly90yyarl9wdw84nx11ffbnx";
   };
 
   buildInputs = [