This avoids trying to use layers for which vbos are not ready yet

Normals are not defined for travel moves, so we need to enable the array
after drawing them, not before

This way we know that the error does not come from the setter itself but
from the validator and can report in a better way the error

This is probably a good call for the future... Changing lights in
glpanel/gcview won't break stlview anymore

We enable it by default but don't want it for stls

Hopefully this was not a hack before :)