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.