video: set pixel format for macOS

Change-Id: I0d3c7a0ec4a09504b25e36e23c9b44c02aff3d71
diff --git a/src/media/video/osxvideo/video_device_impl.mm b/src/media/video/osxvideo/video_device_impl.mm
index 0f4d4d2..940deb0 100644
--- a/src/media/video/osxvideo/video_device_impl.mm
+++ b/src/media/video/osxvideo/video_device_impl.mm
@@ -115,6 +115,7 @@
     params.input = params.name;
     params.framerate = rate_;
     params.format = "avfoundation";
+    params.pixel_format = "nv12";
     params.width = current_size_.first;
     params.height = current_size_.second;
     return params;