Fixed glX symbols not being found because was X11 not being linked on Linux, rewrote the FindSFGUI.cmake script.
Moved BREW.hpp back into the public includes so users can use it as a base for their own Engine implementations, marked BREW's Create methods as override.
Implemented non-legacy renderer, moved all GL code out of Renderer and into the Renderer implementations, added checks for GL errors around all GL calls, fixed broken GLX linking.