Fix GCC optimization-level bug.

Fix:
	src/graphics/screen.c
		-When compiled under anything but `-Ofast`, the engine's SDL
		screen would refuse to display anything; the bug was tracked
		down to `ZBuffer_t` code introduced to `screen.c` in 62cf154.
		`memset` was being used -- incorrectly -- to set
		`g_zbuffer->buf` values.
		-Reimplement `clearZBuffer()` with a generic for-loop that sets
		the values "manually."