Drag preview does not consider camera position

**Godot version:**

3.2.2

**OS/device including version:**

MacOS 10.15.5

**Issue description:**

set_drag_preview() does does not account for camera movement currently. It seems to take mouse position from viewport only.
Problem described in detail in https://github.com/godotengine/godot/issues/11281

Currently I solve this issue with an offset by hand like this:

```
var preview = Control.new()
var preview_unit = from_control.duplicate(true)
preview_unit.rect_position = camera.global_position-_position-(get_viewport_rect().size/2)
preview.add_child(preview_unit)
set_drag_preview(preview)
```
This is however not obvious and should either be documented or camera movement should be considered when positioning the preview

**Steps to reproduce:**
Run project. Drag and drop picker.

**Minimal reproduction project:**

[drag_and_drop 2.zip](https://github.com/godotengine/godot/files/4895246/drag_and_drop.2.zip)