mapi_abi: Override 'hidden' and 'handcode' attributes using polymorphism.

mapi_abi: Override 'hidden' and 'handcode' attributes using polymorphism.

Previously, the ES1, ES2, and shared GLAPI printers passed a list of
function names to the base class constructor, which was used by the
_override_for_api() function to loop over all the API functions and
adjust their 'hidden' and 'handcode' attributes as appropriate for the
API flavour being code-generated.

This patch lifts the loop from _override_for_api() into its caller,
and makes it into a polymorphic function, so that the derived classes
can customize its behaviour directly.  In a future patch, this will
allow us to override the 'hidden' and 'handcode' attributes based on
information from the XML rather than a list of functions.

Tested-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>