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/capture_pump.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include <cstdint>
+#include <memory>
+
+namespace api::capture_pump {
+
+    struct Frame {
+        // packed 24bpp RGB, width * height * 3 bytes
+        std::shared_ptr<uint8_t[]> pixels;
+        uint64_t timestamp = 0;
+        int width = 0;
+        int height = 0;
+    };
+
+    // the graphics layer has one capture slot per screen, so concurrent waiters would steal
+    // each other's frames; everything that captures goes through here to keep it serialized
+    bool capture_direct(int screen, std::shared_ptr<uint8_t[]> &out, int divide,
+            uint64_t *timestamp = nullptr, int *width = nullptr, int *height = nullptr);
+
+    // a screen carries one stream at a time; false when another connection already holds it
+    bool claim_screen(int screen);
+    void release_screen(int screen);
+}