Selected active strip highlights effect inputs and input channels for
multicam strips. This was done as 3rd pass in `draw_seq_strips()`.
4th pass was highlighting strip when doing solo preview.
Now this is added to a column of functions in `draw_seq_strip()`.
Highlighting effect input and solo preview is done when drawing actual
highlighted strip, but this is still done via alpha blending.
Highlighting multicam has to be done as alpha blending since this is
done by drawing rectangle over whole channel.
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