Yeah, it looks like that NVidia system is returning values in studio range. Which is what the current Wine algorithm was doing (where black is R = G = B = 16 = 0x10 and white = R = G = B = 235 = 0xeb). I have no idea why though. Mine certainly doesn't.
I've been able reproduce those NVidia results on my install. There's a setting in the NVidia control panel that allows you to set "video color settings" so that the 'Dynamic range' is 'Limited (16-235)'.
So it seems, on Windows, it is a driver implementation and can be influenced by settings. So I guess this MR is effectively changing "that setting" in Wine from 'Limited (16-235)' to 'Full (0-255)'.
But it also makes Wine consistent with `ffplay`, `gst-play-1.0` and dxvk; which all use "Full Range" (i.e. they all render black as 0).