Test the software rendering pipeline's results by comparing them to the GPU

[adroitwhiz]

Expected Behavior

There should be a way to automatically ensure that the CPU and GPU renderer produce the same results.

Actual Behavior

Currently, the only way to ensure that the CPU and GPU render the same image is by testing their "touching sprite" and "touching color" behavior in the integration tests. Unfortunately, this allows many bugs to slip through because they're not covered by existing integration tests, and ensuring ahead of time that that specific behavior should be tested for is tough.

There's already a CPU rendering "playground" that renders out the entire stage as the software rendering pipeline "sees" it and then compares it to the same image on the GPU. It's currently a bit flaky, but it could be fixed up and potentially leveraged to ensure that at most only a few pixels differ between the CPU and GPU's rendition of the stage.

This is blocked on:

• Fix texture sampling calculations for CPU pipeline #479 - these are the bare minimum changes necessary to ensure that at the very least, a basic comparison test will pass.
• Switch from Chromeless to Puppeteer for tests #537 - this should be merged before attempting to write any new tests, so they don't have to be ported to Puppeteer later.