Replace 1D c_offset with 2D c_offset_name

If the mixed space is not square, c_offset redeclares variables since it
only looks at the current arg position and the size of the map.  Fix
this by using a 2D naming scheme (the same as c_map_name).
Merge pull request #276 from OP2/fix/non-square-mixed-offsets

Replace 1D c_offset with 2D c_offset_name