Fix invisible link label text

When using the experimental pango rendering a previous change
introduced a bug that caused the link label text to be invisible.
When measuring the text ranges of the link pieces the clip
rectangle isn't set properly so we better ignore it and stick to
the boundaries that are passed in.

This fixes BL-666.

Change-Id: I74792d2e2e114cc133b4afb7bac27f17fa025617