Improve Regression Testing

[PTaylour]

Motivation

• getting occasional non-deterministic fails with current setup
• currently not straight forward to add new tests, need to create HTML each time.
• no visible feedback on why tests fail (only percentage diff)
• inaccurate screenshot timing (can't guarantee we're always taking pictures of the same frame)

This PR

• adds cypress to run the end to end tests
• moves the video context test code into the tests themselves
• uses a snapshot library that provides visual diffs on fail
• adds a test harness for ensuring frame accurate snapshots

Progress

• write test harness
• port some tests
• port all tests
• remove old regression tests
• document: running, roadmap, updating
  - [ ] resolve screen shot issue with Proposal: Visual Regression / Screenshot Diffing cypress-io/cypress#495 (comment) didn't work. Waiting on cypress bug

Known issues

• bug means we can't use the GUI of cypress as captures screenshots at different size to CI Snapshots sometimes captured at smaller size jaredpalmer/cypress-image-snapshot#67
• GUI version (when enabled) creates separate screenshots depending on how you start the tests Snapshots for running all specs don't use individual spec snapshots jaredpalmer/cypress-image-snapshot#61
• cypress are working on their own image snapshot feature, so we may want to jump ship to that at some point

[PTaylour]

@richski This is now done. BIG PR.

[richski]

Made a few minor non-code (README & comments) formatting tweaks - d7d443e.

Looks good!