use vlc_fourcc_GetChromaDescription to get the bits per pixel

A few values may differ from the original switch, but they seem more correct.

smem and video_format_Setup() disagree on the kind of information put in the video format i_bits_per_pixel. The former counts the number of bytes needed, the latter really counts the average bits per pixels. The latter seems more correct. For example, the i_bits_per_pixel is used to compute the buffer size in bluraySendBackgroundImage() or the stride in screen/win32, xwd.