Establish code style for GStreamer plugin

[hack-s]

The code currently mixes some code styles: 2 space intention, 4 space intention, curly brackets on the same line or next line etc. I think it would be good to reformat the code to a common style. We should adopt whatever is the most standard code style for C / GStreamer or projectM.

[hack-s]

this would be the code style for GStreamer: https://gstreamer.freedesktop.org/documentation/frequently-asked-questions/developing.html?gi-language=c#what-is-the-coding-style-for-gstreamer-code