gtkcssshadowvalue: Don't double-apply the alpha to shadows

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.