Rect2.clip returns incorrect results when using Rect2 containing negative size values

[Anaxagor14]

Godot version:
Godot 3.1 beta

OS/device including version:
Windows 10 Home build 17763.195

Issue description:
Using Rect2.clip returns incorrect results if one of the Rect2 objects has a size containing negative values.
Other systems using Rect2 seem to work fine with them, like draw_rect.

Steps to reproduce:
Here is a simple code example:

            var rectA = Rect2(0, 0, 8, 8)
            var rectB = Rect2(2, 2, 8, 8)
            var overlap_AB_OK = rectA.clip(rectB) # Returns Rect2(2, 2, 6, 6)
            var rectC = Rect2(10, 10, -8, -8) # Equivalent to rectB
            var overlap_AC_wrong = rectA.clip(rectC) # Returns Rect2(0, 0, 0, 0)
            var overlap_AC_OK = rectA.clip(rectC.abs()) # Returns the correct value, same as overlap_AB_OK 

[akien-mga]

Related to #23711.