Qt apps are oversaturated on wide gamut macOS displays

Generally, all Qt apps produce oversaturated colours when run on DCI-P3 displays, which are most new Mac displays.

This is due to colorspace matching being disabled for Mac backing stores (app colorspace = window = display colorspace). This produces oversaturated results as any UI rendered is not compensated from sRGB colorspace as with other apps.

Proposed patch attached forces app to be rendered in sRGB colorspace. This may be an overkill, however this should be on by default, with Qt / other platforms lacking colorspace controls.