Refactor scanlineRender(), rem drawMatrixLines().

Add:
	src/graphics/graphics.c
		-Refactor `scanlineRender()` to modify the parameter `Point_t *`s
		directly, without defining double variables with equal values.

Rem:
	src/graphics/matrix.(c, h)
		-Remove deprecated `drawMatrixLines()`.
Fix Point_t * bugs, sphere fencepost error.

Fix:
	src/graphics/geometry.c
		-The `Point_t *` passed to both `scanlineRender()` and `drawLine()` were
		pointers to allocated `Point_t`, and problems ensued because they were
		directly modified by each of the functions; add `COPY_POINT()`
		statements to each.

	src/graphics/geometry.c
		-`addSphere()` had an off-by-one-error in its `addTriangle()` loops,
		running one more time than was necessary.

	src/graphics/screen.c
		-Set the values of empty (ie, undrawn) `ZBuffer_t.buf` coordinates to
		`-1`, because a negative color value is nonsensical, so as to allow easy
		on-the-fly identification of a previously undrawn pixel.
Fix torus creation.

Add:
	src/graphics/geometry.c
		-Fix `addTorus()`, which had several fencepost-error `addTriangle()`
		calls.
Fix scanlineRender() artefacts.

Add:
	src/graphics/graphics.c
		-Fix the stray lines that `scanlineRender()` occasionally drew to the
		screen by flooring each vertex `Point_t *`'s y-coordinate.