Clip and Image Metadata

# Open Effects Proposal for Standard Change

Please read the contribution [guidelines](https://github.com/ofxa/openfx/wiki/Extending-OpenFX-Guidelines#submit-a-proposal-or-bug-report-to-review) first.

## Standard Change Workflow
- [X] Create proposal as issue (you're doing this now!)
- [X] Tag this issue with `standard change` tag
- [X] Identify subcommittee: at least one plug-in vendor, and at least one host
- [ ] Discuss the idea in this issue
- [ ] Write new or updated code and doc
- [ ] Publish updates as a pull request (ideally on a `feature/PROPOSAL-NAME` branch)
- [ ] Make sure that PR references this issue number to keep them in sync
- [ ] Discuss and review code in the PR
- [ ] Meet all requirements below for accepting PR
- [ ] When subcommittee signs off and other members don't have any further review comments,
maintainer merges PR to master which closes PR and issue

## Requirements for accepting a standard change:
- [ ] Header files updated
- [ ] Documentation updated
- [ ] Release notes added
- [ ] Compatibility review completed
- [ ] Working code demonstrated with at least one host and one plugin
- [ ] At least two members sign off
- [ ] No further changes requested from membership

## Summary

This proposal aims to add metadata read/write capability to plugins.

## Motivation

Some plugins want to be able to get the file path and frame number of the media that sourced the current clip and/or frame.
Other plugins may want to query information about the lens used, or the stack of previous effects applied. The ability to add metadata to the output may also be useful.

## Problem

Currently there is no way to query anything about input images to an effect came from, such as the name or path of the source clip.

## Impact

This is a new feature, embodied as a new set of properties. It dos not affect any existing suites or properties.

This feature is entirely optional. A host that does not implement it will simply appear to have no meta data to an effect. There is no requirement for an effect to make use of it at all.

## Documentation Impact

The documentation for this feature could be embodied entirely in the header file additions.

## Stakeholders

Plugin writers should benefit the most from this change.

## Discussion