VDP: use bit 0 for Mode H40

(cherry picked from commit 9742047901c98d06ebde886f57bd3341babe24a5)
GFX: shadow/highlight also affects backdrop (streetracer)

(cherry picked from commit f56f23485f4af9df7dafa05daebb69c9c34ac8fe)

Conflicts:
	gfx.cpp
VDP: implement sprite collision flag (fix strider2)

(cherry picked from commit 4c40545d9075092b077576eebd94419b18508905)

Conflicts:
	mem.h
	vdp.cpp
	vdp.h