engine: improve GL state management, add support for GL sampler objects, add libtess2 and implement in polymer in place of the GLU tesselator, drop dependency on libGLU, improve GL debug output (in debug builds), fix various GL resource leaks

This should prevent a few more state changes, and address issues arising from rendermode changes wiping our state but not any of our accounting data for said state.

Because libtess2 seems to be abandoned and because there were a few issues I needed to fix anyway, I went ahead and made a custom single-file header-only version for improved performance.