[3.x] Add a 2D version of the dynamic split screen demo

[BenjaminNavarro]

Here is an updated version of the dynamic split screen demo project that now includes a 2D implementation along the existing 3D one.

It took me some time to figure out a way to do this in 2D as cameras and viewports behave differently compared to 3D. In the 3D version there are only 2 additional viewports, one per camera. In 2D, I had to create a third viewport to hold all the game content and share its 2d_world with the two other viewports. I don't know if there is a better way to do this.

The shader is the same for both 2D and 3D but the scrips are all slightly different.

I also reworked the 3D scene so that it's more similar to the 2D one (due to its 3rd viewport limitation)

See #762 (comment)

Bugsquad edit: This closes #650.

[BenjaminNavarro]

After the offline suggestion from @NathanLovato to use only 2 viewports in 2D as well (didn't thought about it) I refactored the project a little to make it simpler and more consistent across the 2D and 3D versions.

I pushed a new commit so you can inspect both versions if you want, I'll squash them later after the reviews.

The main changes are:

1. The node hierarchy is the same for both versions (except for Node2D/Spatial and Camera2D/Camera of course)
2. I merged 2d/camera_controller.gd and 3d/camera_controller.gd into camera_controller.gd. I detect if the script runs for a 2D or 3D scene, set the mode enum and use it in a couple of places to handle the difference between the two modes. Since most of the code was the same I think it's better now considering future maintenance and updates but I can roll back to two different scripts if you think that would be clearer to users

[aaronfranke]

This PR has a few conflicts and needs to be rebased.

In general a 2D version of this demo seems nice.

[BenjaminNavarro]

Rebasing done on 3.x, nothing had to be changed