Add SDL screen unit tests.

Add:
	src/unit_tests.c
		-Add `testScanLineRender()` and `testZBuffering()`.

	src/graphics/screen.c
		-Significant additions to the `screen` module were necessary to
		accommodate unit-tests for scanline-rendering, z-buffering, and more.
		-Add `createZBuffer()`, `clearZBuffer()`. Expose `g_zbuffer` to global
		scope.
		-Add `writeZBufferToFile()` and `readZBufferFromFile()`.
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.
Merge branch 'unit_test_addition'

Added unit tests for the SDL screen, by taking advantage of the `ZBuffer_t`
introduced in @7d87a82.