Clean up and misc fixes

[ceyusa]

This pull request is a preparation for the video frames rendering as gl textures in the player example:

• isolate the video sinks setup in a function
• add gst logging capabilities to gstreamer player backend, rather than use print macros
• specify a specific commit id to webrender, since we are using git head, but an update will break our example app
• use glib and gstreamer preludes, this is a general clean up recommended by @sdroege
• the player example eats all the available memory since a great big image leak
• instead of keeping a vector with pushed frames from player example, add a custom queue with only three frames
• as we are using Arc'ed data, there's no need to return a reference

[ceyusa]

@ferjm +?