glcpp: Add a flag to the parser state to indicate GLES.

glcpp: Add a flag to the parser state to indicate GLES.

This can be triggered either by creation of a GLES context (with
api == API_OPENGLES2) or else by a #version directive with version
value 100 or with a string of "es" following the version value.

There's no behavioral change with this commit—just preparation for ES-specific
behavior in the preprocessor in the future.

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