This function is used to fixup variables from QNames to stack frame
indexes at stylesheet build time.
param
vars List of QNames that correspond to variables. This list
should be searched backwards for the first qualified name that
corresponds to the variable reference qname. The position of the
QName in the vector from the start of the vector will be its position
in the stack frame (but variables above the globalsTop value will need
to be offset to the current stack frame).