Fix filter shaders when rectMode is applied; add tests

[davepagurek]

Resolves #6602

Changes:

• Uses rect() instead of image() to avoid rectMode compatibility issues
• Removes a test that was reliant on the rect implementation
• Added tests to make sure results look the same regardless of the renderer, rect mode, image mode, and blend mode
• Fixed some other minor bugs that came up in the test cases
• Small refactor to give framebuffers their own filter camera so we don't create a new camera every frame

Screenshots of the change:

Before:

After:

PR Checklist

• npm run lint passes
• Inline documentation is included / updated
• Unit tests are included / updated

[davepagurek]

@perminder-17 mind giving this a review when you get the chance?

[perminder-17]

@perminder-17 mind giving this a review when you get the chance?

Thank you @davepagurek for allowing me to review your code. It seems we're using plane instead of rect to address the issue with rectMode. I believe this is a good approach.