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.
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.
Merge pull request #79 from foolip/cue-background-box

Remove weird properties on the WebVTT cue background box
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
Merge pull request #104 from foolip/position-auto

Let position have a default "auto" value, which depends on align
Merge pull request #78 from foolip/position-size-order

Reverse the order of position and size
Merge pull request #98 from silvia/positionAlignAuto

Introduce a positionAlign value of "auto"

Conflicts:
	webvtt.html
Merge pull request #99 from silvia/positionAlignComputed

Introduce "computed text position align"

Conflicts:
	webvtt.html
Merge pull request #101 from silvia/positionFixesRegion

Apply position and positionAlign fixes also to regions.

Conflicts:
	webvtt.html
Merge commit 'b4e4d826bf721b6d8cbba5716fc2a4290e87a2e1' into revert-regions
Merge commit '5b8f235719c004b8f09ba5de1e54cfada46d6762' into revert-regions

Conflicts:
	webvtt.html
Merge commit 'bfb16dfd234088aa33504f0eff3b4b42439a9249' into revert-regions
Merge commit 'c0da0c2044829e5c554959a6d2f4e8ba9a598e6e' into revert-regions

Conflicts:
	webvtt.html