AtlasEngine: Make Direct2D/3D and Present1 configurable (#16939)

This implements `SetForceFullRepaintRendering` and adds a new
`SetGraphicsAPI` function. The former toggles `Present1` on and off
and the latter allows users to explicitly request Direct2D/3D.

On top of these changes I did a minor cleanup of the interface,
because now that DxRenderer is gone we don't need all that anymore.

Closes #14254
Closes #16747

## Validation Steps Performed
* Toggling Direct2D on/off changes colored ligature support ✅
* Toggling Present1 on/off can be observed in a debugger ✅
* Toggling WARP on/off changes GPU metrics ✅

---------

Co-authored-by: Dustin L. Howett <duhowett@microsoft.com>

src/cascadia/UnitTests_SettingsModel/SerializationTests.cpp

@@ -107,8 +107,6 @@ namespace SettingsModelUnitTests
                 "trimPaste": true,
 
                 "experimental.input.forceVT": false,
-                "experimental.rendering.forceFullRepaint": false,
-                "experimental.rendering.software": false,
 
                 "actions": []
             })" };