Use vertex arrays to draw in DrawPolygon for OpenGL.

This should introduce a minor performance boost for slow CPUs in
OpenGL mode. If it doesn't, it shouldn't be any slower. It also puts
us a little closer to supporting GLES as well.

This introduces several new pgl function pointers that are from
GL 1.1. I do not check for their existence because no driver out in
the wild in active use doesn't support 1.1.