Implement z-buffering.

Add:
	src/graphics/screen.(c, h)
		-Add `ZBuffer_t`, to contain the SDL screen's z-buffer.
		-Make `drawPixel()` simply plot points in the global z-buffer, while
		`renderScreen()` calls `plotPixel()` to draw the buffer's pixels on the
		SDL screen.
Exchange the names of drawPixel() and plotPixel().

Add:
	src/graphics/screen.(c, h)
		-Rename `drawPixel()` to `plotPixel()`, and `plotPixel()` to
		`drawPixel()`, because the new names arguably better represent the
		nuanced difference between the functions' functionality.