Fix mouse position in sub-viewports

[KoBeWi]

Fixes #60390

As I was debugging the issue, I discovered that the problem lies in Viewport's method to get mouse position:

godot/scene/main/viewport.cpp

Lines 1100 to 1102 in b5f20a4

	Vector2 Viewport::get_mouse_position() const {
	return gui.last_mouse_pos;
	}

It's returning the last recorded mouse position. The problem is that sub-viewports (or at least the 2D editor viewport) never updates this value, so it was always returning (0, 0).

This was originally changed in:
c7b4dca
Not sure why. I copied the previous code and it fixes the issue.

[akien-mga]

CC @Sauermann

[Sauermann]

@akien-mga thanks, I have seen this PR. I will revisit this PR after #67791, which touches related behavior.

[Sauermann]

The approach of this PR is to fix the problem for one additional layer of SubViewports.
In #71768 I aim to solve the problem even for deeply nested Viewports.

[Sauermann]

I believe, that the reason, why this PR has failed the checks, was fixed in #71972.

[akien-mga]

Superseded by #71768.