waveform scope: add option for luma display

[dtorop]

In waveform scope can now either show RGB channels or luma. When luma option is on, make the RGB channel buttons insensitive.

RGB channels

Luma

Note that this is luma (Y') not relative luminance (Y). Until add an option to display linearized RGB data, it doesn't make sense to show linearized Y, not least as the jump from seeing non-linear RGB channels to linearized relative luminance is disconcerting.

Calculate either RGB channels or luma waveform, not both. This makes sure that RGB waveform calc is as fast as it was before luminance calculation added. Luma calculation is ~15% faster than RGB calculation. This does require reprocessing the pixelpipe when turn on/off the luma option.

Future improvements could be:

• Add option to show linearized waveform (and rgb parade) data
• When linearized option is on, show luminance rather than luma
• Add option to show luma/luminance in addition to RGB channels in RGB parade mode
• Show luma/luminance in histogram

Also:

• Rename vs_prof parameter to profile in non-vectorscope _process() functions. The vs_prof name was a result of copying the name from vectorscope process.
• Update some comments and general tidying.

[dtorop]

@Donatzsky @VBK7 You both mentioned interest in a luminance waveform display, so curious how this seems. Caveats:

1. As mentioned above, this is luma, not relative luminance. Luma seems to make sense until there is an option to show linearized waveform data.
2. This is currently only in waveform mode, not rgb parade or histogram.

And a query: Is luma/luminance display helpful for determining clipping? Is there ever a case one wants to be sure that luminance rather than individual channels are clipped?