EEVEE: Add new experimental "EEVEE Next" option

This is supposed to hold the latest improvement from the EEVEE rewrite branch.
Note that a restart is necessary in order for the engine to appear.

The registration code is a bit convoluted as it needs to be after the WM_init.

source/blender/render/RE_engine.h

@@ -253,6 +253,7 @@ void RE_engine_render_context_disable(struct RenderEngine *engine);
 /* Engine Types */
 
 void RE_engines_init(void);
+void RE_engines_init_experimental(void);
 void RE_engines_exit(void);
 void RE_engines_register(RenderEngineType *render_type);