Panel control with Stylebox set to Draw Center has 1px error

[realkotob]

Godot version:

Godot 3.2.3 stable.

OS/device including version:

Both GLES3 and GLES2. Windows 10. Nividia 930M.

Issue description:

Panel stylebox does not always respect its own rect size, and children it will bleed by 1-2 pixels. This happens when zooming in/out the editor, or when running the game with the stretch mode set to anything other than None.

I expect the Panel stylebox to stick to its specified size regardless of screen resolution or zoom.

In this below picture you can see the edges of the Panel that is supposed to be completely hidden by the ColorRect in front of it.

Note that toggling "snap_controls_to_pixels" in project settings also did not help.

Steps to reproduce:

• Create Panel node
• Add Stylebox and set to Draw Center
• Add child ColorRect set to Full Rect layout (to help visualize the error)
• Scroll in/out in the editor (or launch the game and resize the screen)

Minimal reproduction project:

panel_style_flicker.zip

[Calinou]

Related to #46493.