Implement support for FSAA in combination with post-processing (#15392)

- Actually it's MSAA I think, or perhaps the terms are equivalent
- I've made it fit into the existing Irrlicht architecture, but that has resulted in code duplication compared to my original "hacky" approach
- OpenGL 3.2+ and OpenGL ES 3.1+ are supported
- EDT_OPENGL3 is not required, EDT_OPENGL works too
- Helpful tutorial: https://learnopengl.com/Advanced-OpenGL/Anti-Aliasing, section "Off-screen MSAA"
- This may be rough around the edges, but in general it works

irr/include/IRenderTarget.h

@@ -26,6 +26,7 @@ enum E_CUBE_SURFACE
 };
 
 //! Interface of a Render Target.
+/** This is a framebuffer object (FBO) in OpenGL. */
 class IRenderTarget : public virtual IReferenceCounted
 {
 public: