The Managed Resource Editor fails to show content of resource files created with the Core Designer

**Visual Studio Version**:
Visual Studio 2019 >=16.5.0 Preview 1.0.
(The necessary Core-Designer ResourcePicker feature has not been merged at the time of creating this issue, though - see _Steps to Reproduce_).

**Summary**:
With the WinForms Core Designer, local resource files (later also project resource files) for Forms or UserControls are added, whose images content is not created by the runtime but by the out-of-proc Designer Surface Process. The images that end up in the resource files are thus based on the corresponding .NET Core type and cannot be displayed by the Managed Resource Editor.

**Steps to Reproduce**:
Since the feature is not yet publicly available in the Visual Studio previews, the easiest way to repro is to use the attached sample project to track the bug.

[ManResourceEditorRepro.zip](https://github.com/dotnet/project-system/files/3823464/ManResourceEditorRepro.zip)

1. Load the attached project in Visual Studio.
2. Expand the branch Form3.cs
3. Double-click on Form3.resx to start the Managed Resource Editor to show the content of the file.

**Expected Behavior**:
The Managed Resource Editor should show the image content of the resource file.

**Actual Behavior**:
The Manage Resource Editor shows this.

**User Impact**:
Users can neither show or edit Images, which have been added to the resource files via the .NET Core WinForms designer.