Small refactor. Add glXFunctionIterator, which derrives from

Small refactor.  Add glXFunctionIterator, which derrives from
glFunctionIterator and is used by GlxProto.  The difference between the two
iterator classes is that glXFunctionIterator skips functions that the GLX
protocol code does not care about.

Replace all the remaining occurances of glParameter::p_count_parameters and
glFunction::count_parameters with the count_parameter_list.

Add GlxProto::size_call to generate the C code to calculate 'compsize'.

These trivially modify the generated code.