Subtitle: TTML support

[3052]

I have the below file, the subtitles will not load:

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'out.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf60.16.100
  Duration: 01:38:48.75, start: 0.000000, bitrate: 3957 kb/s
  Stream #0:0[0x1](und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(progressive), 1200x720 [SAR 1:1 DAR 5:3], 3822 kb/s, 24 fps, 24 tbr, 19200 tbn (default)
    Metadata:
      handler_name    : USP Video Handler
      vendor_id       : [0][0][0][0]
      encoder         : AVC Coding
  Stream #0:1[0x2](zho): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 127 kb/s (default)
    Metadata:
      handler_name    : USP Sound Handler
      vendor_id       : [0][0][0][0]
  Stream #0:2[0x3](eng): Subtitle: ttml (stpp / 0x70707473), 1 kb/s (default)
    Metadata:
      handler_name    : USP Subtitle Handler