Fix pango fontmap threading issue.

Fix pango fontmap threading issue.

Since pango v1.32.6 `pango_cairo_font_map_get_default` returns a per thread `fontmap`. But since we're allocating this statically on the first call to `rrd_init`,
if we then try to access it again via another thread glib will get stuck in an infinte assertion loop.

Instead use `pango_cairo_font_map_new()` when creating the `fontmap`. This will not create a per thread version and glib will be happy.

Also added some extra locking whenever using the `im->layout` pango structure that uses the `fontmap` internally as a precaution.