Fix #614 Add video scaling options

Freeseer now supports converting video signals from one display
resolution to another. Technically we already supported this, but there
were no scaling options so it wasn't very useful.

The following scaling options are now available when recording video:

- No scaling
- 240p (320x240)
- 360p (480x360)
- 480p (640x480)
- 720p (1280x720)
- 1080p (1920x1080)