Reenable support for UI Elements that use Render Targets (#2352)

* Re-add support for UI Elements that use Render Targets
* Move LyShine pass request from Atom's MainPipeline.pass to project's
* Make all dynamic draw contexts in LyShine draw to pass directly without the need of draw list tags
* Remove local RPI changes that are no longer needed
* Prevent crash if LyShine gem is enabled but its custom pass hasn't been added to the main render pipeline
* Revert to default UI pass if the LyShine pass has not been added to project's main render pipeline

Signed-off-by: abrmich <abrmich@amazon.com>

Gems/LyShine/Code/Editor/EditorWindow.cpp

@@ -1547,6 +1547,20 @@ AssetTreeEntry* EditorWindow::GetSliceLibraryTree()
     return m_sliceLibraryTree;
 }
 
+AZ::EntityId EditorWindow::GetCanvasForCurrentEditorMode()
+{
+    AZ::EntityId canvasEntityId;
+    if (GetEditorMode() == UiEditorMode::Edit)
+    {
+        canvasEntityId = GetCanvas();
+    }
+    else
+    {
+        canvasEntityId = GetPreviewModeCanvas();
+    }
+    return canvasEntityId;
+}
+
 void EditorWindow::ToggleEditorMode()
 {
     m_editorMode = (m_editorMode == UiEditorMode::Edit) ? UiEditorMode::Preview : UiEditorMode::Edit;