Fixed mat4_set_frustum clipping issue.

Noticed mat4_set_frustum and mat4_set_perspective were not working as expected. Notably, they were not setting the far clipping plane. I have updated mat4_set_frustum to match the man pages (http://www.manpagez.com/man/3/glFrustum/). In particular ->m31 should be ->m21 and ->m32 was missing the zfar term.