Backdrop gradients don't take gamma into account

Since cairo has no notion of color spaces, it naively interpolates the given RGB values to render gradient backdrops: https://gitlab.xfce.org/xfce/xfdesktop/-/blob/master/src/xfce-backdrop.c#L195

This however is the incorrect way as the color values provided by the user are sRGB values and the interpolation needs to happen in linear color space. See https://blog.johnnovak.net/2016/09/21/what-every-coder-should-know-about-gamma/#gradients for more details.

If we agree that this is worth fixing, I could come up with a PR that renders the gradient the right way, bypassing cairo's gradients.