[iip] Underline and overline need to clear glyph ascenders and descenders (#75)

r12a has just created a new issue for https://github.com/w3c/iip:

== Underline and overline need to clear glyph ascenders and descenders ==
Indian language text has some signs called as Matras which sometimes join above the shirorekha or below the normal baseline.

Applications should ensure that the underline and overline when getting rendered, should adaquetly be taken into consideration. Chrome browser seemed to be perfectly rendering the underline and overline feature by breaking the same where a matra/vowel sign occurs. Firefox also handles it a bit differently by appropriately lowering or heightening the respective lines. 

Please view or discuss this issue at https://github.com/w3c/iip/issues/75 using your GitHub account