v4l2-renderer: add support for alpha.

Pass alpha value to the underlying v4l2-renderer-device.
vsp-renderer-device: add support for alpha channel.

Add support for global alpha.

N.B. VSP1 cannot support both global alpha and pixel alpha simultaneously.
vsp-renderer: open subdev via v4l2_subdev.

Avoid a possibility to open the same subdev nodes multiple times.
vsp-renderer: scaler work in progress..