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/launcher/options.cpp

@@ -1743,6 +1743,21 @@ static const std::vector<OptionDefinition> OPTION_DEFINITIONS = {
         .setting_name = "1",
         .category = "Companion & API",
     },
+    {
+        // APIStreamEnable
+        .title = "API Video Stream Server Enable",
+        .name = "apistream",
+        .desc = "Serves the mirrored screen as a video stream, on the API port plus two; "
+            "alternative to API screen capture. Requires -api.\n\n"
+            "http://host:apiport+2/stream.mjpg - MJPEG\n\n"
+            "http://host:apiport+2/stream.h264 - H.264\n\n"
+            "Parameters: screen (0-3), fps (1-60, default 30), q (1-100, default 70).\n\n"
+            "Example with -api 1337: http://host:1339/stream.h264?fps=30&q=70\n\n"
+            "VIEW ONLY - touch input still requires -api. "
+            "No password protection or encryption of any kind; video sent in the clear!",
+        .type = OptionType::Bool,
+        .category = "Companion & API",
+    },
     {
         .title = "Enable All IO Modules",
         .name = "io",