glcpp: Accept pointer to GL context rather than just the API version

glcpp: Accept pointer to GL context rather than just the API version

As the preprocessor becomes more sophisticated and gains more optional
behavior, it's easiest to just pass the GL context pointer to it so that
it can examine any fields there that it needs to (such as API version,
or the state of any driconf options, etc.).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>