QColorSpace story for QRhi

What's the HDR and QColorSpace story? All GL-specific colorspace support added in 5.x goes missing in 6.0 by default. (since QOpenGLFramebufferObject and co. are not in use anymore, the OpenGL paint engine may be replaced, etc.)

May not be that urgent now that we will not be having an RHI-based QPainter backend yet.

Note that this has nothing to do with true HDR output (on HDR-capable displays), but is rather about QImages and texture-based render targets using high precision formats.