Change the syntax of Region headers

[zcorpan]

See https://www.w3.org/Bugs/Public/show_bug.cgi?id=18657

It is suggested to change the syntax here to use REGION blocks before the cues (consistent with STYLE), instead of headers. I suppose this would also be an opportunity to change from = to : so that would be consistent with cue settings.

If we use a block it would be possible to allow linebreaks between settings here, if people think that would be nice to be able to do. e.g.

WEBVTT

REGION
id:fred
width:40%
lines:3
regionanchor:0%,100%
viewportanchor:10%,90%
scroll:up

REGION
id:bill width:40% lines:3 regionanchor:100%,100% viewportanchor:90%,90% scroll:up

...

Apple has shipped support for regions, so need to check with them if this change is OK. @dwsinger ?

[silviapfeiffer]

Eric Carlson's statement at the recent FOMS was that if we want to change it, now is the time.

[eric-carlson]

I support this, making region consistent with style is a great idea.

[zcorpan]

Need to fix #222 first to do this in a sane way. (Depends on: #246)

[silviapfeiffer]

👍