Tutorials all can't show video on macOS

Submitted by Sebastian Dröge @slomo

Link to original bug (#790242)

Description

See https://github.com/sdroege/gstreamer-rs/issues/58 , same applies to the C tutorials.

The problem is that for osxvideosink and glimagesink we need to run a NSRunLoop on the main thread on macOS. Our patched GLib version does that when you run a GMainLoop (upstream GLib doens't!), but the examples are only using gst_bus_timed_pop_filtered() so that also wouldn't help.

Using a GMainLoop would make the tutorials quite a bit more complicated, more API would have to be introduced, there would always be callbacks, etc.

Not sure what to do about this!