fn color_replace(
img: &mut Frame,
from: [f32; 3],
to: [f32; 3],
tol: f32,
soft: f32,
)Expand description
Key out colours near key (0..255) by their Cb/Cr distance, with spill removal. No edge shrink —
that needs neighbourhood taps; shaders::CHROMA_KEY does it on the GPU.
Blend every pixel within tol of from towards to, fading out over tol .. tol + soft.
Distances are normalised so 1.0 is the full black-to-white diagonal, matching the GPU body.