Fix for bug 6936752 Tamil text gets truncated on right-hand side

The getTextRunAdvances() method wasn't accounting for the true width of
the text. On closer examination, in Tamil the clusters consist of a
number of glyphs each of which has a nonzero advance (in some other
scripts, the first glyph in the cluster has an advance, and others are
effectively zero). Previously, we were just using the advance of the
first glyph in the cluster. This patch changes the behavior to sum the
advances of all the glyphs within the cluster.

Change-Id: I77a51235f4bb0dfaa72cbb920a8c3b217ad25404