tnl: add some comments in render_line_loop code
tnl: add some comments in render_line_loop code

And remove '(void) flags' line which is not needed.

Reviewed-by: Jose Fonseca <61b5a29db2c0650f7c8510f93eece059c1b09320@vmware.com>
Reviewed-by: Sinclair Yeh <326bbc75ae5821d6c8d1817b260a4d8d0fdf1f63@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler