Add some basic functional testing

[jonathanhogg]

Add some basic functional testing

[jonathanhogg]

Good news. I've merged support for offscreen rendering and added a few simple tests that create an engine, load a script that creates an output image, execute that script and then check that the output image is correct. See #37. This is now part of the GitHub workflow – so these basic engine functional tests will be executed on commit.

At the moment I'm only generating solid green outputs and confirming that the resulting bytes are correct. Next up would be adding support for comparing interesting output images to see how close the rendering is to the reference image.

[jonathanhogg]

As I've been writing documentation, I've been doing some diagrams in Flitter to explain certain functions' output (see builtins.md). I've now added tests to recreate these diagrams and compare them against the reference ones included in the documentation. Next step will be to add some images into the tutorial and do the same there.

Sadly all of this uses only 2D rendering, so I'll need to do some 3D examples. Probably the smart thing to do is to slightly rework all of the examples to be able to dump their output to an image file and then include reference images for each of the examples.

[jonathanhogg]

Tutorial images added and successfully tested.

[jonathanhogg]

I've got this working for the examples as well. So I'm calling this done.