Tl next tc fix

Dear Joker developers,

This pull request aims at reducing the number of drawing calls per frame. OpenGL rendering can slow down a lot when the number of calls is too large on some graphic cards. OpenGL traces produced by [apitrace](http://apitrace.github.io/) revealed that a large number of texts drawings were not producing any pixel modifications. It turns out that these are invisible people labels. These commits add checks on the position of the labels before drawing.

On the test video "tmwtga", these commits reduce the number of calls per frame from 3650 to 2300. On another video I am using, they reduce the number of calls from 7900 to 2200.

Thanks for your attention !