Redefine ZBuffer_t.

Add:
	src/graphics/screen.h
		-Redefine `ZBuffer_t` with a `buf` member variable that's
		allocated on the heap, as opposed to the stack, to allow
		variable dimensions per run as required by the
		`src/unit_tests.c`.

	src/graphics/screen.c
		-Rewrite `createZBuffer()` to account for the changes detailed
		above.
		-Add `freeZBuffer()`, as a lengthy routine of `free`s is now
		necessary to fully deallocate a `ZBuffer_t`.