Update unit-test CSVs. Fix ZBUffer_t I/O bug.

Add:
	test/*.csv
		-Update existing unit-tests to reflect the addition of lighting,
		amongst other features.

Fix:
	src/graphics/screen.c
		-Fix a bug in `writeZBufferToFile()` and
		`readZBufferFromFile()`, which caused `equalZBuffers()` to
		occasionally fail (incorrectly) due to floating-point errors.
Add horizontal-line and ZBuffer_t I/O unit-tests.

Add:
	src/unit_tests.c
		-Add `testDrawHorizontalGradientLine()` and `testZBufferIO()`.