Previously scale_cascaded() assumed the whole source frame arrived in a
single sws_scale() call, and the dispatcher only routed full-frame calls
to it. A partial input slice fell through to ff_swscale() on the parent
dispatcher context, whose scaler state (c->desc) is never initialized in
cascade mode, causing a NULL dereference / crash.
Top-down sliced output is bit-exact with full-frame scaling; bottom-up
matches swscale's pre-existing (non-cascade) slice behaviour for
subsampled intermediate formats.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
Michael Niedermayer
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