gtkcssshadowvalue: Don't double-apply the alpha to shadows
It turns out that when we were painting the shadows, we painted the them with the base color once, which contained the alpha, and then blurred it and used it as a mask for the fill, which has the fill again. To fix this, always paint the base surface with full alpha. The existing code applies the blur conditionally sometimes in weird ways, so the code shuffling fix may not look correct, but be assured it is. If the blur happens, the new cr we return has the *default* color applied, which is fully opaque black, which works perfectly against the A8 surface. The fallback spinner code needs some modification, since it is intentionally using the alpha to paint the lobes which are "in the past". Since we shouldn't be hitting this fallback path very often, we use a temporary group and paint it with paint_with_alpha, even though it is slow.
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