avfilter/tonemap*: add ICtCp based tonemap method

[gnattu]

This adds ICtCp based tonemap to OpenCL, Metal and CUDA based tonemap filters, and will use the mode as the new default. On GPUs with lower specs, OpenCL and CUDA implementation will default to relative luminance mode. The Metal implementation does not have a performance tradeoff mode and will always default to ICtCp mode.

This also fixed attachment related issue on Metal when output is 10bit by always setting the attachement values.

The ICtCp mode's desat algorithm is different from other modes. The coefficient applied to the chroma is defined by a Gaussian Function in the form:

$$f(I) = \exp\left(\frac{-\left(I-\frac{L_{\text{target}} - p_{\text{desat}}}{2}\right)^2}{2L_{\text{peak}}}\right)$$

Where $I$ is the ICtCp intensity linearized by ST2084 EOTF, $p_{\text{desat}}$ is the desat parameter, $L_{\text{target}}$ is the target peak luminance (1.0 for SDR) and $L_{\text{peak}}$ is the HDR peak luminance for current frame.

The current curve is tuned to preserve more color detail to preserve natural looking than to aggressively turning bright colors into white, and is different from the initial curve implemented in #429.

Original

desat=0.5 in #429

desat=0.5 current

This curve is not a standardized practice and is purely my own creation, which means it could lead to unpleasant results for certain cases I've not tested on.

Changes

Issues

Fixes #429