shaders/h274: correctly infer comp model values

The ITU-R H.274 spec specifies what values to infer when the num_model_values is lower than the required number of components.

As an aside, we also never correctly adapted the model values for chroma planes. We also still don't correctly adapt them for subsampled planes, but that is a problem for a future commit.

Lastly, the current behavior technically breaks on negative scale values (which underflow to huge unsigned int values). Fix this by switching from uvec to ivec.

Fixes #187 (closed)