swscale/output: fix bilinear yuv2rgb chroma interpolation

These functions were divided into two special cases; one assuming that
uvalpha == 0, and the other assuming that uvalpha == 2048. This worked fine
for simple 2x chroma upscaling but broke for e.g. yuv410p, non-centered chroma,
or other special cases that involved non-aligned chroma filters.

Fix it by instead dividing this check into two cases, a uvalpha==0 fast path
and a uvalpha>0 general path. Instead of (A+B)/2 the general path now multiplies
in the true uvalpha weight.

I tried preserving the old fast path for the case of uvalpha == 2048, but this
was significantly slower in practise versus having just one general path.
However, we still need a uvalpha == 0 path for the unscaled case.

Fixes: ticket #5083
Signed-off-by: Niklas Haas <git@haasn.dev>
Sponsored-by: Sovereign Tech Fund

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