glapi/gen: Gather API version info across aliased functions.

glapi/gen: Gather API version info across aliased functions.

gl_XML.py's gl_function class keeps track of an entry_point_api_map
property that tracks, for each set of aliased functions, which ES1 or
ES2 version the given function name first appeared in.

This patch aggregates that information together across aliased
functions, into an easier-to-use api_map property.

Future patches will use this information when code generating
_mesa_create_exec_table(), to determine which set of dispatch table
entries should be populated based on the API.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>