api: h.264 video stream (#876)

## Link to GitHub Issue or related Pull Request, if one exists
fixes #875

## Description of change
Adds `-apistream`, an optional HTTP video stream of the mirrored screen.
It listens on the API port +2.

Two endpoints, sharing the same `screen`, `fps` and `q` parameters:

    /stream.mjpg    JPEG frames, for clients with no container support
/stream.h264 H.264 annex-b, for an app driving MediaCodec or
VideoToolbox itself

One encoder per connection, fed by a per-screen pump that always hands
over the newest frame, so a slow reader drops frames instead of building
a backlog. `capture.get_jpg` behaviour is unchanged.

Additional documentation for developers:
https://github.com/spice2x/spice2x.github.io/wiki/Video-Stream

## Testing

src/spice2x/api/stream_format.h

@@ -0,0 +1,38 @@
+#pragma once
+
+#include <cstddef>
+#include <functional>
+#include <memory>
+#include <string>
+
+#include "capture_pump.h"
+
+namespace api {
+
+    // writes bytes to the client; false once the connection is gone
+    using StreamSend = std::function<bool(const void *, size_t)>;
+
+    // one wire format, instantiated per connection so it can keep encoder state across frames
+    class StreamWriter {
+    public:
+        virtual ~StreamWriter() = default;
+
+        StreamWriter(const StreamWriter &) = delete;
+        StreamWriter &operator=(const StreamWriter &) = delete;
+
+        // value for the HTTP Content-Type response header
+        virtual std::string content_type() const = 0;
+
+        // for formats that open with an init segment; runs once before any frame
+        virtual bool begin(const StreamSend &send) { return true; }
+
+        virtual bool write(const StreamSend &send, const capture_pump::Frame &frame) = 0;
+
+    protected:
+        StreamWriter() = default;
+    };
+
+    // null when the path does not name a format this build supports
+    std::unique_ptr<StreamWriter> make_stream_writer(
+            const std::string &path, int quality, int fps);
+}