Hook AVFormatContext.io_open so HLS/DASH segment fetches and other
nested FFmpeg I/O go through stream_http when libcurl is available.
This lets segments inherit the connection reuse, HTTP/2 multiplexing,
mid-stream resume and conditional compression of the top-level stream.
Falls back to FFmpeg's default I/O for non-HTTP URLs and when the curl
backend is not built in.
Introduces stream_http, an internal libcurl-driven backend for http://
and https:// URLs. Runs all transfers on a dedicated curl multi thread
with a producer-side ring buffer, and exposes HTTP/2 multiplexing,
QUIC/HTTP/3 (when libcurl supports it), HSTS and TCP keep-alive.
Should generally be more stable and faster than FFmpeg HTTP/1.1 impl,
additionally connections are kept-alive between files open, so if you
open playlist of network files and navigate through them, it will re-use
the same connection.
Build is gated on the new 'libcurl' meson option (auto). When disabled
or unavailable, mpv silently falls back to FFmpeg's HTTP implementation.
Kacper Michajłow