Add support for SSA (v4+) MarginL, MarginR, MarginV style

[szaboa]

Reopening google/ExoPlayer#10169.

This PR is about adding support for MarginL, MarginR, MarginV from both the Style and Dialogue lines. I've used VLC as reference.

• Non-zero margins defined in Dialogue lines takes priority over the margins in Style lines.
• In case we have a {\pos} override, then no margins will be applied neither from Dialogue or Style (same as in VLC).
• In case we have a {\an} override, then VLC ignores the Dialogue margin but applies the Style margin which makes no sense for me, so in our case I've just ignored both Dialogue and Style margins (same as {\pos} case).

Suggestion: maybe we could update the https://storage.googleapis.com/exoplayer-test-media-1/ssa/test-subs-position.ass subtitle file with these new lines, then no need for a new media in media.exolist.json.

[peerless2012]

Add ass feature for exoplayer use Java code mabe a huge work。