Comment 6 for bug 1573470

xwininfo/xdpyinfo seems to confirm the red and blue channels are reversed for GLX windows:

  visual:
    visual id: 0xb4
    class: TrueColor
    depth: 24 planes
    available colormap entries: 256 per subfield
    red, green, blue masks: 0xff0000, 0xff00, 0xff
    significant bits in color specification: 8 bits

versus non-GLX windows:

  visual:
    visual id: 0x28
    class: TrueColor
    depth: 24 planes
    available colormap entries: 256 per subfield
    red, green, blue masks: 0xff, 0xff00, 0xff0000
    significant bits in color specification: 8 bits

However the problem is not as simple as changing a pixel format. Xmir is running in rooted mode, so we only get to choose one visual for the whole display at start-up. So we need to find a way to force GLX to use the correct visual, or to force XComposite to mix the colours correctly. Because it's really out of Mir's control...