assrender: Incorrect detect size of surface for subtitle rendering when playback anamorph video

Submitted by RussianNeuroMancer @RussianNeuroMancer

Link to original bug (#625685)

Description

Look at attached screenshots. I get incorrect rendering in Totem player on Ubuntu Maverick. Correct rendering - it's mplayer.
Sample: http://sharebee.com/65fa39aa
When aspect ratio of video stream not equal with aspect ratio in properties of this video stream in media contatiner (anamorph video) rendering ass subtitles always incorrect. For example real size of video in this sample file is 1440x1080, but it's playback like 1920x1080.