Let position have the same interpretation for LTR and RTL text

Use the 'ltr' code paths in rendering, remove the 'rtl' paths,
then simplify the switches by merging similar conditions.
Let position have a default "auto" value, which depends on align
Merge branch 'revert-position-size-align' into position-size-align

Conflicts:
	webvtt.html
Merge branch 'position-size-align' into minvtt
Make this an unofficial draft