Sometimes, HTTP sources require a lot of seeking during probing / header
parsing (especially for formats like MXF). Currently, we need to completely
tear down and re-establish the connection most times this happens, which puts
a lot of stress on the network stack and also results in transmission of
possibly many unnecessary bytes.
This patch adds an option to allow FFmpeg to request partial ranges during
the initialization stage. This is done until the initial request size is fully
read, after which we fall back to the normal behavior (i.e. infinite streaming
via an unbounded request).
The usefulness of this is limited without also specifying -multiple_requests 1,
since otherwise there is little point to requesting partial ranges to begin
with. (However, it is semantically independent, so we keep it that way.)
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