renderer: perform debanding first

I'm not entirely sure I'm happy with this commit for two reasons: Firstly, because it disables the optimization where we only deband croppd video regions. Secondly, because it prevents pass merging of the debanding pass with the main video merging pass, which is something I really wanted to have working. On the plus side, it means we no longer deband after plane scaling (in the case of e.g. luma prescalers), which is definitely a performance bug.

I'll have to think more about what to do about these problems.

Fixes: #230 (closed)