Rem IMAGE_WIDTH/HEIGHT macros, add size detection.

Add:
	src/globals.h
		-Remove hard-coded `IMAGE_WIDTH` and `IMAGE_HEIGHT` screen-size
		macros.

	src/graphics/screen.c, makefile
		-Add screen size detection with functions from the `X11`
		library.
Add configureTestingEnvironment().

Add:
	src/unit_tests.c
		-Add `configureTestingEnvironment()`, to set global variable
		values accordingly for `unitTests()` to use.

	test/testConfiguration.csv
		-Add unit-test configuration file used by
		`configureTestingEnvironment()`.
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`.
Merge branch 'feature/screen_size' into develop

Add cross-computer screen-size adaptation.