renderer: do color management before frame mixing

Solves several issues:

1. Avoids re-running tone mapping shader when nothing about MT params changed
2. Avoids bad mixing artefacts when mixing in pre-TM HDR colorspaces
3. Avoids re-running 3DLUT/ICC on every vsync
4. Avoids re-running all of the above when re-rendering static still image from mixer cache

Mostly this is just a performance win across the board, although it does require the user to enable preserve_mixing_cache to avoid having to re-render the entire pipeline when e.g. the ICC profile changes.