I'm not sure what shaders or FBO's have to do with this. The simplest and fastest implementation would be plain old OpenGL 1.x rendering a rectangle (and later rendering 4 x textures). Nothing more is required.
Let's not make the Unity mistake of getting deep in shader and FBO magic that could actually be achieved with much simpler OpenGL code (no shaders or FBO extensions).
I'm not sure what shaders or FBO's have to do with this. The simplest and fastest implementation would be plain old OpenGL 1.x rendering a rectangle (and later rendering 4 x textures). Nothing more is required.
Let's not make the Unity mistake of getting deep in shader and FBO magic that could actually be achieved with much simpler OpenGL code (no shaders or FBO extensions).