Changes to the handling of "position" and "positionAlign".

Introduce both an "auto" value for "position" and "positionAlign"
settings which depends on the "text alignment".

Introduce a "computed text position" and "computed text position
alignment" algorithm such that these can be set by script through VTTCue.

This results in some simplifications in the rendering algorithm, in
particular we now don't need positionSet and positionAlignSet variables
any longer.
AFAICT this was the original issue in https://www.w3.org/Bugs/Public/show_bug.cgi?id=19178, namely that for snap-to-lines, all the previous positioning is ignored, instead positioning the bounding box of the text in both axes in a manner similar to CSS background-position. I think that bug should be reopened.