Clean up and enhance scissor and viewport testing

Add one new test which ensures that initial scissor setting is correct and
that scissor does not change when canvas is resized.

Add black borders around canvases in scissor and viewport tests so that
debugging is easier.

Clean up unnecessary logging, typos, one unnecessarily included script.
Merge pull request #760 from Oletus/improve-scissor-tests

Clean up and enhance scissor and viewport testing