4.3: background not cleared in compatibility mode on subviewport when environment background_mode is canvas

[tripodsan]

Tested versions

• reproducible in 4.3 compatibility mode; works in 4.2.2

System information

MacOs v4.3.stable.official [77dcf97]

Issue description

I have a split screen setup with a (transparent) subviewport and a background on a parallax background. additionally, I want to use glow, so I set environment.background_mode to Canvas. In the Forward+ renderer, this works. but with the compatibility renderer, it seems that nothing clears the background, because the environment has the canvas background, and the other solid items are on a different canvas layer (parlallax). this causes the subviewport rendering to overdraw.

Steps to reproduce

(btw, I initially thought this was an issue due to the glow...but it isn't)
setup a screne with a background on a different canvaslayer, and a subviewport with transparent background.
also set the environment background_mode to canvas:

2024-11-08_13-31-18.mp4

here's a demo: https://tripod.itch.io/godot-subviewport-test

Minimal reproduction project (MRP)

https://github.com/tripodsan/godot-subviewport-test

[clayjohn]

Fixed by #100388