Expose GL render profiling numbers to ZScript

[JPL]

The GL renderer tracks things like rendered_lines, rendered_flats, rendered_sprites for stat and benchmark purposes. It would be useful if these numbers were accessible from ZScript, entire benchmarking processes could possibly be driven from script. Object has some functions that operate in a global-ish context like this, but I notice their ZScript declarations says "These really should be global functions..." so maybe there's a better place for it.

[Graf Zahl]

These numbers are renderer specific. We got 3 renderers and all of them use different variables to track their work.
Even more than the subsectors, this has no place at all in the play code, as it is highly specific to user setup.

[JPL]

Any use cases for this would likewise be highly specific to user setup; if the user is running another renderer the function would return zeroes as with "stat renderstats". Definitely not thinking of gameplay mods with this.