Texture Generators (#1779) * Add ResourceGenerator * Add ResourceGeneratorContext * Add TextureGenerator * Add Bitmap * Add Sandbox.Utility.ForAsync * Add IPopupEditor<T>, EditorUtility.OpenControlSheet looks for this interface when creating a popup * Rename ControlSheetPopup to PopupEditor, * Add Color.Rgba16 (half based color) * Managed can override compiling of any resource * Add Texture.GetBitmap * Add transient filesystem in editor * Add Texture.IsError * Add Margin.IsNearlyZero * Fix asset picker ignoring AdditionalTypes * Add Widget.GetDescendants<T> * Add MainThread.Wait() * Expose TypeDescription.Order