Pango: update interface for PangoContext and PangoFontMap.

Pango: update interface for PangoContext and PangoFontMap.

The pango_ft2_get_context() function is deprecated.  Using
pango_context_new() followed by pango_context_set_font_map()
accomplishes the same task.

Fortunately, defining PANGO_ENABLE_BACKEND enables pango_context_new()
and pango_context_set_font_map() for Pango versions older than 1.22.

The recommended function, pango_font_map_create_context(), requires
Pango 1.22, so avoid using it unless we absolutely have to.