vf_colorspace: Interpret unspecified color range as limited range

This is the assumption that is made in pixel format conversion do
throughout the code (in particular swscale), and BT-specifications
mandate.

Add a warning to inform the user that an automatic selection is being
made.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
