gtktextlayout: fix for right margin with RTL text

When a RTL paragraph is not set to wrap, the right margin is not respected
because of the margins counted twice so we replace
display->width by PIXEL_BOUND (extents.width), the same quantity
without the margins.

https://bugzilla.gnome.org/show_bug.cgi?id=741702
gtktextlayout: fix to quiet a warning

Gcc complains about a possible use of a not initialised
widget ref ( which can't happen in reality )

https://bugzilla.gnome.org/show_bug.cgi?id=741702