With this change we are consistent about how the pango layout
we use is created.
Now it always comes from a pango_layout_new() call on a pango context
that comes from gdk_pango_context_get().
Previously the pango layout used for painting came from a call
to pango_cairo_create_layout(), which required a global called
"current_cr" (a cairo drawing context), which is only valid
during redraw (painting). Since it was only valid during
painting, this source could not be used for pango layout
creation for the measuring code.
