color: add get_windows_scrgb

The Windows flavor of scRGB color space for HDR cannot easily be represented with the parametric image description factory. The reference white level is unknown, and the target color volume / MDCV is unknown yet negative and greater than one color channel values must be supported. Also gamescope wants to support this without implementing set_luminances request.

I thought of writing a new factory for enumerated image descriptions, but decided to reduce the boilerplate by making windows_scrgb just another feature flag in itself. If we need more enumerated image descriptions, extending this way seems fine, all the support advertisements are in the wp_color_manager_v1 interface anyway even if they are specific to a factory.

It is not possible for a compositor to indicate Windows-scRGB as an output or a surface's preferred image description in this design. I cannot see the use for that given how much of the image description is essentially unknown, especially the reference white level.

Having Windows-scRGB is useful for supporting HDR applications and games written primarily for Windows. It is not meant for "native" Wayland applications, they can get better and more reliable results by using the parametric image description factory.

This is an alternative proposal to !78 (closed).

cc @frog, @swick, @Zamundaaa