So you want to be able to design your whole level as a giant texture? Why are you using 3D?
In 2D, the solution is simple, use a StaticBody with a CollisionPolygon, and the editor will let you edit the polygon to trace the borders of your level directly over it. There are ways to code it to make it automatic, but you need to write this yourself, more headscratching.
Doing this in 3D is less straightforward if you want to use a texture. Instead, the 3D engine expects collision meshes, so it's easier to design this mesh into a 3D editor such as Blender for example, because you can then import it and generate the collider from it.
If you manage to generate the outlines with a script tough, you can also generate this mesh collider from code (but still needs headscratching :p)