More test fixes

[davepagurek]

• Adds pixelDensity(1) to texture tests so that running tests on a high dpi display doesn't cause them to break
• Replaces the image diff library with something more custom so that we can try to accommodate pixel shifts better
• Removes old unused browser-based visual test runner

[davepagurek]

@sproutleaf I've skipped the param validation tests for now, we should look into those and get them running again.

@limzykenneth looks like all the (non-skipped) tests pass on CI now! we have the green checkmark back!!