vio6: Convert color format into YCbCr in RPF

Some units such as SRU require YCbCr color format for
input images.
PRELIMINARY: vio6: Apply the super resolution processing into resizing

This inserts the super resolution processing into resizing which
would be performed with alpha-blending through vio6_setup_blend().
In this preliminary implementation, the super resolution processing
can be applied only when color format of the 1st input image is YCbCr.
vio6: Allow to specify SRU level value through a shell environment variable