Always get the Pango context through GTK+ API rather than creating it
from a Cairo context to always get a context set up for the screen.

This fixes font size issues when using Cairo rendering.

Patch imported from upstream Mercurial (3596/070c7eb483bf).

git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@5690 ea778897-0a13-0410-b9d1-a72fbfd435f5