This is an attempt to recreate a glow effect in Godot 4's 2D (specifically Compatibility Mode but Vulkan also applies).
We are trying to get a similar effect to Glow in Godot 3, so we use some shader code from that: https://github.com/godotengine/godot/blob/3.5/drivers/gles3/shaders/effect_blur.glsl
This approach uses a SubViewport to contain any textures that want to glow.
The game world nodes need to place a texture into the Subviewport and then use a RemoteTransform2D to keep it aligned.
Currently if one glowed sprite is behind another sprite, the glow will not be clipped.
Looking for solution!
If solution found, then we can proceed with better glow effect.
This solution will probably be short-term. However, it may end up being faster than using HDR on low-end devices, or more customizable.