Add testDrawLine(). Fix zbuffer-reading bug.

Add:
	src/unit_tests.c
		-Add `testDrawLine()`.
		-Add `ASSERT_EQUAL_SCREEN()` macro, for the `ZBuffer_t` unit tests.

Mod:
	src/graphics/screen.c
		-Have `writeZBufferToFile()` and `readZBufferFromFile()` check the width
		and height of a `Zbuffer_t` written to a file, as opposed to just their
		product.