[rbp] Don't consider half-SBS/TB 3D modes to have double the framerate

Fix for this bug report:
http://forum.xbmc.org/showthread.php?tid=140518&pid=1306106#pid1306106

This change only has an effect when CONF_FLAGS_FORMAT_SBS/CONF_FLAGS_FORMAT_TB is set, so only affects Pi.

On Pi, the 3D modes look like:
960x1080@24 for SBS
1920x540@24 for TB

(note: framerate is not reported as 48)

So, doubling a video's framerate (which is ~24 fps) means that the 24p TV modes are rejected for being too low, and so you get a 50Hz mode.

The fix removes this unwanted doubling of framerate.