[membrane_ivf_plugin] Fix plugin

[Noarkhh]

The plugin hasn't been touched in a long while, we should:

• rename the plugins to not have the Element part
• use autodemands
• add support for VP8 and VP9 stream formats
• fix a bug with endianness in frame_count field in the headers
• deal with counterfeit and fraudlent fixtures
• add file sink seeking in order to write the number of frames to the header based on the actual number of frames that were processed
• refactor