Remove Platform::sceneEglConfig() and Platform::sceneEglContext()
With per-screen rendering, every output may have different EGLConfig. Having a single global EGLConfig doesn't work out well.
This change removes Platform::sceneEglConfig(). It's used primarily to create the global share context. In hindsight, the global share context can be created without EGLConfig as it's never made current.
EGL_NO_CONFIG_KHR is part of EGL_KHR_no_config_context extension, which is supported by both Mesa and NVIDIA driver so it should be safe to make it mandatory.
Edited by Vlad Zahorodnii