Fix ghosting when using background_tint under GNOME+Wayland

The problem was that on Wayland if the buffer contains pixels with alpha
< 1 they are blended with something, even if the window is opaque. Under
mutter that something was the previous frame, under sway it was the
background/whatever is under the window.

So when blending the tint color, use a blend mode that results in opaque
pixels.

Fixes #5605

docs/changelog.rst

@@ -42,6 +42,8 @@ Detailed list of changes
 
 - Wayland KDE: Fix abort when pasting into Firefox (:iss:`5603`)
 
+- Wayland GNOME: Fix ghosting when using :opt:`background_tint` (:iss:`5605`)
+
 0.26.4 [2022-10-17]
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