input: use input format for category

The input format is fixed for the lifetime of the decoder thread. The output format is not always available as the packetiser or the decoder may delay the determination of their output format until they start outputting packets.

The downside is the assumption that input and output category are the same. This assumption is already enforced by an assertion, and relied heavily upon by the ES output for track selection, so it is hardly a problem.

Fixes #21975 (closed)