[Cin] Some tests on blending

[Andrea paz]

I tried again to test Color Space plugin and noticed some peculiarities:
If I set Input: BT709 and Output BT709, and also BT2020 -> BT2020;
then the result is not clipped. Any other combination of Input/Output
leads to a clipped result instead. How can this be explained? Perhaps
the input is not clipped (apart from BT601 which is always clipped),
but every transformation is always clipped. 709 -> 709 and 2020 ->
2020 does not perform transformations and thus remains unclipped.
This argument can perhaps also be carried over to the “YUV Color
Space” tool: if we leave the source color space unchanged then we can
keep the pipeline without clipping (if it was already unclipped to
begin with), if we convert to any other color space then clipping will
occur.