fix(FEC-12991): Left-positioned captions are centered on Firefox (#704)

issue: we override the positionAlign on VTTCue with no excepted values, so Firefox ignores other values and keep its default value - 'auto'
fix: changed the configuration to use the excepted values, (and keep the old ones for backward compatibility)

solves: FEC-12991

docs/configuration.md

@@ -580,7 +580,7 @@ var config = {
 >   lineAlign: string, // ['start', 'center', 'end']
 >   align: string, // ['start', 'center', 'end', 'left', 'right']
 >   position: number, //[0 .. 100]
->   positionAlign: string, // ['start', 'center', 'end']
+>   positionAlign: string, // ['line-left', 'center', 'line-right']
 >   snapToLines: boolean, // [true, false]
 >   vertical: string, //['', 'lr', 'rl']
 >   size: number //[0 .. 100]

src/track/text-track-display.js

@@ -664,13 +664,15 @@ class CueStyleBox extends StyleBox {
     switch (align) {
       case 'start':
       case 'left':
+      case 'line-left':
         textPos = cue.position;
         break;
       case 'center':
         textPos = cue.position - cue.size / 2;
         break;
       case 'end':
       case 'right':
+      case 'line-right':
         textPos = cue.position - cue.size;
         break;
     }