Add support for SMPTE-TT base64-encoded subtitles images.

Created by: TobbeEdgeware

Added support for embedded base64-encoded SMPTE-TT image element.

Tested with http://vm2.dashif.org/dash/vod/testpic_2s/img_subs.mpd

Compared to before imscJS integration, I saw the following differences:

The name space xmlns:tts="http://www.w3.org/ns/ttml#styling" needed to be corrected (was #style#).

Of the three variants on using tts:extent and tts:origin: http://vm2.dashif.org/dash/vod/testpic_2s/img_subs_px.xml http://vm2.dashif.org/dash/vod/testpic_2s/img_subs_percent.xml http://vm2.dashif.org/dash/vod/testpic_2s/img_subs_div.xml the two first still work, but the third does not since it specifies tts:extent and tts:origin in the divelement, and this is not allowed by TTML.