Subtitles: Add option to render subtitles at source video resolution.

Fixes #60
VideoRenderers: Set the video size through a setter.

Don't set the aspect ratio to the video size anymore when it is not known, this case is already handled by GetVideoSize().
VMR7: Use GetVideoSize() instead of correcting the aspect ratio explicitly.
VideoRenderers: Fix GetNativeVideoSize returning an invalid aspect ratio.

The multiplication was invalid, the documentation specifies that only the aspect ratio needs to be returned.
Update Release.txt