GLInterface: Destroy GLWin

Everything is now safely tucked inside each individual GLInterface.
X11_Util: Don't specify a background pixel

This causes flickering when resizing the window, which looks horrid
and we shouldn't do it.
Add back X11 support to EGL

Refactor the EGL backend to provide a platform separation here, which is
better abstracted away than the old EGL/X11 implementation.