This branch implements support for transforming SDR (sRGB) to a HDR enabled output, while allowing HDR content to stay intact.

Roughly what it does is, when HDR mode is enabled via the experimental property, all the stage views that represents outputs where HDR actually managed to be enabled starts to composite in a linear variant of the target color space (which will be BT.2020) via an intermediate framebuffer. Each thing being painted should then transform its pixels to fit in this color space, using an EOTF, a color space mapping matrix and luminance adaptation.

Here is the merge request