If Atom isn't initialized and able to produce a ViewportContext, the Editor would crash. This attempts to make the initialization fail a bit more gracefully and fixes the crash in the cases I've tested.
* Move Draw2d.h to Include folder
* Initial phase of UI Canvas Editor rendering with Atom
* Simplify Draw2d by removing BeginDraw2d/EndDraw2d which is no longer needed
* Fix compile errors for non-unity builds