In an HBoxContainer a Viewport with size 0 in a stretch-enabled VIewportcontainer does not render until first resize

[Grosskopf]

Godot version:

3.1.2

OS/device including version:

OpenSuse Tumbleweed

Issue description:

is related to #26187 and can be easily fixed setting the viewport size to something good in the first place but maybe this helps someone figure out #34805

Steps to reproduce:

make a 3d scene and a 2d scene, put a viewportcontainer in a hboxcontainer and make them resize and enable stretch,
put the viewport in the viewportcontainer and put the viewports size to 0,0, notice how it doesn't render on first frames,
resize the window to resize all the containers
notice how it now works :D

Minimal reproduction project:

testproject.zip

[KoBeWi]

Still valid in a511a26

[Calinou]

and can be easily fixed setting the viewport size to something good in the first place

This is fixed in master by #48814.