Fix TextEdit.get_rect_at_line_column returning negative pos even though cursor is in viewable area of the control

[MJacred]

Sister PR of #81352 (for Godot 3.x)

When both are merged, #81349 can be closed

Production edit: Fixes #81349

Before fix:

textedit-bug_godot-4.webm

After fix:

after.webm

[MJacred]

@AThousandShips: done, ready for review

[Paulb23]

Changes look okay to me, would be good to add a unit test to capture this behaviour

[MJacred]

EDIT: aaah! because get_pos_at_line_column returns bottom side of the line (totally forgot)

---

I would do it, but this does not make any sense in my head: if you drag the mouse from column 1 to 7 you should get his is. But apparently you even land in line 1, not 0... Similar confusion with other uses of get_pos_at_line_column in test_text_edit.h

text_edit->set_text("this is some text\nfor selection");
text_edit->grab_focus();
MessageQueue::get_singleton()->flush();

SEND_GUI_MOUSE_BUTTON_EVENT(text_edit->get_pos_at_line_column(0, 1), MouseButton::LEFT, MouseButtonMask::LEFT, Key::NONE);
SEND_GUI_MOUSE_MOTION_EVENT(text_edit->get_pos_at_line_column(0, 7), MouseButtonMask::LEFT, Key::NONE);
CHECK(text_edit->has_selection());
CHECK(text_edit->get_selected_text() == "for s");

[AThousandShips]

Style looks good!

[akien-mga]

Thanks!