vo_gpu: split --linear-scaling into two separate options

Since linear downscaling makes sense to handle independently from
linear/sigmoid upscaling, we split this option up. Now,
linear-downscaling is its own option that only controls linearization
when downscaling and nothing more. Likewise, linear-upscaling /
sigmoid-upscaling are two mutually exclusive options (the latter
overriding the former) that apply only to upscaling and no longer
implicitly enable linear light downscaling as well.

The old behavior was very confusing, as evidenced by issues such
as #6213. The current behavior should make much more sense, and only
minimally breaks backwards compatibility (since using linear-scaling
directly was very uncommon - most users got this for free as part of
gpu-hq and relied only on that).

Closes #6213.

etc/builtin.conf

@@ -40,6 +40,7 @@ cscale=spline36
 dscale=mitchell
 dither-depth=auto
 correct-downscaling=yes
+linear-downscaling=yes
 sigmoid-upscaling=yes
 deband=yes