The H.264 Constrained Baseline Profile (CBP) is a subset of both the
Main Profile and the Baseline Profile. In principles, a hardware
decoder that supports either of those can decode CBP content. As it
happens, Main is supported by all VDPAU drivers, and Baseline is not.

So favor map CBP to MP for now. Hopefully in the future libvdpau will
offer an explicit choice for CBP.

This fixes bug 757.
Signed-off-by: Anton Khirnov <anton@khirnov.net>