Don't use subpasses when we're using Canvas background mode in Mobile

[BastiaanOlij]

When using the Canvas background mode we're copying our 2D render result into our 3D buffer, then rendering 3D, do post processing, and finally copy the end result back into our 2D buffer.

In this setup we can't use subpasses when rendering 3D as we're reading from, and eventually writing to, our 2D render target.
This PR adds a check so the copy to 3D buffer, and copy back to 2D buffer, happen in separate passes instead of in subpasses.

• Fixes Attempted to use the same texture in framebuffer attachment and a uniform (set: 0, binding: 0), this is not allowed. Error continuously spams upon startup #90099

[chrisl8]

I can confirm that the errors are gone with v4.3.dev.custom_build [9e9a6fb] when running the https://github.com/godotengine/godot-demo-projects/tree/master/2d/particles demo.

[clayjohn]

Makes sense!

[akien-mga]

Thanks!