Refactor `membrane_h26x_plugin`

[varsill]

Recently I needed to make some large-scale changes in the membrane_h26x_plugin and the current state of the plugin made it particularily difficult.

The main problems are:

• H264 and H265 parsers seem to be too tightly coupled
• H26x.Parser behaviour defines a set of "ad-hoc" callbacks that were added just because some codec-specific customization was needed so we end up with a flow like H264.Parser.handle_stream_format/4 -> H26x.Parser.handle_stream_format/4 -> H264.Parser.parse_raw_input_format)
• the :error state is not handled in the SchemeParser which make workarounds like throwing a state neccessary