Pango-based font handling: combine display of adjacent characters

Pango-based font handling: combine display of adjacent characters
into a single gdk_draw_layout() where conveniently feasible, after
some work with xtrace revealed this as a major source of pterm's
slow display updates when using client-side fonts.

Ideally we ought to be able to do better. I know exactly what
sequence of X protocol operations I want to see on the wire, but I
don't know how to persuade Pango to generate them.

git-svn-id: svn://svn.tartarus.org/sgt/putty@8558 cda61777-01e9-0310-a592-d414129be87e