Remove weird properties on the WebVTT cue background box

These were added in commits 3d05e2e05c72b01404cf290bf07c7f0f367827a7 and
be09934775d380753873ab9667c70e1b3af43aa3 but don't make sense since the
WebVTT cue background box is a per-cue inline box.
Move the ruby anonymous box logic back to its original place

This was moved in commit 3d05e2e05c72b01404cf290bf07c7f0f367827a7
but seems to belong better in its original place since it creates
boxes and does not merely set properties.
Remove the region nullification from the parser

Scripts can already get cues into the state that this tries to avoid,
and it seems harmless due to how the rendering algorithm works.

https://www.w3.org/Bugs/Public/show_bug.cgi?id=25948
Introduce a positionAlign value of "auto"

"auto" is the default positionAlign setting
It depends on the "align" setting.
Introduce "computed text position align"

allows removing of positionSetAlign in the rendering section
Apply position and positionAlign fixes also to regions.
Improve on some of the wording.
Fix some typos.
Be consistent when referring to "text alignment".
Reverse the order of position and size

The order of these were change in commits
6b7c8613d219890f170a67c1099ca1468702d716 and
c84ec7734114195da3644fdac4793f311aee72fc.
Introducing a position value of "auto".

"auto" is the default value for the position setting.
It depends on the "align" setting.
Introduce "computed text position"

allows removing of positionSet in the parsing section
Remove re-positioning of boxes on snap-to-lines

Boxes are already positioned with x-position and y-position calculation
in step 8 when snapt-to-lines is true.
Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=19178