Patch for Acid3 test 77 (SVGTextContentElement errata)

Hello Ian,

The SVG 1.1 errata item for SVGTextContentElement[1] was moved from draft  
to proposed as per resolution in the June 19 telcon[2].

The errata item in question changes the behaviour for some of the things  
that were tested in test#77 in the acid3 test, more particularly what's  
changed is the getSubStringLength method and on what indata it throws  
exceptions for.

The attached file contains a patch that I believe is correct according to  
the new definition.

It's likely that there will be a publication of the SVG 1.1 errata soon.  
Once that happens, it would be great if the attached patch could be  
included in the acid3 test.

Cheers
/Erik (relates to ACTION-2010)

[1]  
http://www.w3.org/Graphics/SVG/Group/repository/errata/errata.xml#svgtextcontentelement_clarifications
[1] http://www.w3.org/2008/06/19-svg-minutes.html#item04

-- 
Erik Dahlstrom, Core Technology Developer, Opera Software
Co-Chair, W3C SVG Working Group
Personal blog: http://my.opera.com/macdev_ed