Add pixel aspect ratio setting to Titler effect

[herman@…]

Currently, text added with the Titler effect gets too wide if the video is anamorphic, as 16:9 DV or DVD video. Freetype2 can be told to render with different x and y resolutions, and the results look pretty good.

The Titler should either detect the project's pixel aspect ratio, and set its x and y scale accordingly, or there should be a few preset pixel aspect ratios that the user can choose.