screencastService: Use 'screen' encoder profile for openH264

There are "camera" and "screen", the former being the default. Using the later should slightly increase image quality.

---

I just found out about this option by accident and it seems to be a no-brainer to use it here.

See also: gst-inspect-1.0 openh264enc - it's been available since 2014 so should be safe to add.

cc @verdre