Supporting vertical writing captions

[alexandercerutti]

The vertical-writing mode is not supported in the current situation, either in WEBVTT or the upcoming TTML Renderer. In order to support it, we should change the CaptionsRenderer in order to paint them.

Also, we should change the RenderingModifiers in order to support eventual writing modifiers.

For what concerns WEBVTT, these are for sure the affected rows:

sub37/packages/webvtt-adapter/src/Parser/RenderingModifiers.ts

Lines 83 to 89 in 7974fc9

	// case "vertical": {
	// if (isVerticalStandard(value)) {
	// modifier.vertical = value;
	// }
	// break;
	// }

sub37/packages/webvtt-adapter/src/Parser/RenderingModifiers.ts

Lines 134 to 145 in 7974fc9

	case "start":
	case "end": {
	/**
	* @TODO to implement based on base direction
	* base direction is detected with
	*
	* U+200E LEFT-TO-RIGHT MARK ---> start: "line-left", end: "line-right"
	* U+200F RIGHT-TO-LEFT MARK ---> start: "line-right", end: "line-left"
	*/
	break;
	}

sub37/packages/webvtt-adapter/src/Parser/RenderingModifiers.ts

Lines 163 to 167 in 7974fc9

	/**
	* @TODO support vertical in renderer
	* along with a new property "writing mode"
	*/
	// private vertical?: HORIZONTAL | GROWING_LEFT | GROWING_RIGHT = HORIZONTAL;

In order to support vertical writing, this resource might be considered valid. It contains W3C References for CSS, to which we might translate RenderingModifiers: https://w3c.github.io/i18n-drafts/articles/vertical-text/index.en