Using SCREEN_TEXTURE mipmaps in Viewport with Transarent background ruin transparent

[danilw]

Godot version:
3.2.1 stable

OS/device including version:
Linux, Web

Issue description:
Maybe this "expected" and this is not bug, I do not know.

Using textureLod(SCREEN_TEXTURE,SCREEN_UV,1.) in scene that inside of Viewport, when Viewport has Transparent_bg ruin it.
screenshot

Steps to reproduce:
Create Viewport, add scene to it, use mipmap>0 in that viewport.
Link to live web-build with this bug, use mouse+LKM to rotate the camera.

In this Web build on OpenGL browser, only transparent lost, background not ruined.
In this Web build on DirectX browser, bug almost same as on desktop.

DirectX view

Minimal reproduction project:
download link zip