In a previous step of the mesh writing process, the loop `totlayer` was
replaced and a temporary vector is used for layer data instead. Use this
temporary vector to find the UV sub-layers, since the vector and the
CustomData struct in the mesh don't necessarily match. Also use a
local variable for mesh loop count to make it clearer that the mesh
isn't used except at the end.
Thanks to Campbell for finding the source of this error.
de49d18af5