Fix viewport icon rendering on high DPI devices (#1006)

* Clarify ViewportWorldToScreen being in widget space and add DeviceScalingFactor

* -Fix viewport icons being draw wrong on high DPI displays
-Fix loading viewport icons from absolute paths, which

* Address review feedback, fix build

Code/Framework/AzManipulatorTestFramework/Source/ViewportInteraction.cpp

@@ -127,4 +127,9 @@ namespace AzManipulatorTestFramework
     {
         return {};
     }
-} // namespace AzManipulatorTestFramework
+
+    float ViewportInteraction::DeviceScalingFactor()
+    {
+        return 1.0f;
+    }
+}// namespace AzManipulatorTestFramework