The 'mode' setting of a function within an 'enum' element is now used.

The 'mode' setting of a function within an 'enum' element is now used.
Parameters to glX_proto_size.py are now used to determine whether to emit
either get-type function, set-type functions, or both.  When only get-type
functions are emitted, they can optionally alias set-type functions.  This
would be useful if, for example, the two types were in different source
files.

The real work to implement this is in SizeStubFunctionIterator class.  All
of the logic for which functions to iterate and in which order is
implemented there.