video: set pixel format for macOS

Change-Id: I0d3c7a0ec4a09504b25e36e23c9b44c02aff3d71

src/media/video/osxvideo/video_device_impl.mm

@@ -115,6 +115,7 @@ VideoDeviceImpl::getDeviceParams() const
     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;

src/media/video/video_input.cpp

@@ -456,6 +456,7 @@ VideoInput::initAVFoundation(const std::string& display)
 
     clearOptions();
     decOpts_.format = "avfoundation";
+    decOpts_.pixel_format = "nv12";
     decOpts_.input = "Capture screen 0";
     decOpts_.framerate = 30;