Fix zoomed out mode drag chip position when dragging over the scaled iframe

[talldan]

What?

The dragOver event returns some unusual values for clientX and clientY when dragging over a scaled iframe. This results in the drag chip being incorrectly positioned when dragging into the canvas in zoomed out mode.

This PR normalizes those values to ensure the drag chip stays in the correct position.

How?

• Calculate the scaling of the iframe (code stolen from Zoom out mode: scale iframe instead of contents #47004)
• Calculate the amount dragged within the iframe and apply the scaling only to that portion.

Testing Instructions

1. Enable the zoomed out mode experiment
2. In the site editor, edit a template
3. Open the global inserter
4. Drag a block in zoomed out mode

In trunk: The drag chip has the wrong position
In this PR: The drag chip has the wrong position

Screenshots or screencast

Before

Kapture.2023-12-08.at.10.34.41.mp4

After

Kapture.2023-12-08.at.10.32.38.mp4

[aaronrobertshaw]

LGTM!

Thanks for the explanatory comments, they really helped 🙇

Before	After
Screen.Recording.2023-12-08.at.2.42.05.pm.mp4	Screen.Recording.2023-12-08.at.2.50.20.pm.mp4