examples: gl: add a gstgears example

Add a gstgears example to show more clearly how to render a 3d scene from scratch with glfilterapp and use the resulting rendering in a GStreamer pipeline.

The scene is composed by the glfilterapp input texture coming from upstream as the background with the gears from glxgears drawn on top of it, downstream elements can use the glfilterapp output texture containing the full rendered scene.

Compared to the recording example this one uses gst_parse_launch() to make it easier to play around with different pipelines.