Fix incorrect scale reference.
Initial work on multi pass splat support.

When rending without the base layer (as done in any extra passes) we
need to alter how the layers reference the blend maps. The least
horrible solution seemed to be to add more preprocessor defines. Might
be that there's a better way which makes the code less convoluted
though.
Fix issue with fog in multipass.

Disable fog for all passes except the last one, else the fog will stack
up.