shaders/colorspace: re-add correct alpha normalization

This was broken in the past, and also not handled in a very elegant way by the renderer. During the process of fixing the renderer's alpha handling, this logic was removed in 0067206f without a replacement. However, that is also not the correct fix, as the documentation clearly states that the alpha mode is applied by these functions, and other places in the renderer depended on this behavior.

Instead, the correct fix to these functions is to re-add the alpha mode application, but at the correct place in the pipeline, which is on the RGBA ingest, rather than the encoded color. (Basically the opposite place of what the previous logic was doing)

Fixes: 0067206f Fixes: #336 (closed) Fixes: https://github.com/haasn/libplacebo/issues/332 Fixes: https://github.com/mpv-player/mpv/issues/16396 Closes: !709 (closed)