* Enhanced OpenGL backend. refs #822
Depth buffer was added and for that two new options. So a client can enable or disable depth buffer rendering with GLUseDepthBuffer. Additionally the value for the alpha test can be changed with GLAlphaTestValue. Note that SortingStrategy is ignored and some special light things may not work with depth buffer.
The OpenGLe backend is still available. If no one has objections, it may be removed.
Furthermore the static layer function was changed so that it should be possible to mix static with nonstatic layers. And it works without FBO, too. fixes #765