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 #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
Drop 'right' from the list of CSS settings

It is unused since commit 89e29b542c36378718382c1807e2a14fdbcdf16b.
Improve the wording for computed text position alignment

The text position alignment is always set, and the auto value is not
special in the same sense as for line and position.
Use the correct unit for the regions left offset
Merge pull request #108 from foolip/left-unit

Use the correct unit for the regions left offset
Merge pull request #106 from foolip/var-right

Drop 'right' from the list of CSS settings
Merge pull request #107 from foolip/computed-special-value-auto

Improve the wording for computed text position alignment
Add separate enums for lineAlign and positionAlign

https://www.w3.org/Bugs/Public/show_bug.cgi?id=26016
Merge pull request #109 from foolip/align-enums

Add separate enums for lineAlign and positionAlign
Restore size clamping in rendering

https://www.w3.org/Bugs/Public/show_bug.cgi?id=25660

This was removed in commit 3d05e2e05c72b01404cf290bf07c7f0f367827a7.
Text direction (LRT/RTL) now has no effect on position, so the restored
text is somewhat simpler than the original.
Use positionAlign instead of align for maximum size

This currently does the right thing, but will likely need to be revised
after https://www.w3.org/Bugs/Public/show_bug.cgi?id=26020 is fixed.