adjust some of the editor images code for terrain overlays

the adjustments just make it use the more standard SDL utils fcns
for making a transparent (neutral) surface, and they avoid floating
point operations when scaling images

we also fix a bug caused by using "sdl_blit" instead of
"blit_surface", which caused the terrain overlay not to show up

Conflicts:
	src/editor/action/mouse/mouse_action.cpp