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/src/CNullDriver.cpp

@@ -1678,6 +1678,12 @@ ITexture *CNullDriver::addRenderTargetTexture(const core::dimension2d<u32> &size
 	return 0;
 }
 
+ITexture *CNullDriver::addRenderTargetTextureMs(const core::dimension2d<u32> &size, u8 msaa,
+		const io::path &name, const ECOLOR_FORMAT format)
+{
+	return 0;
+}
+
 ITexture *CNullDriver::addRenderTargetTextureCubemap(const irr::u32 sideLen,
 		const io::path &name, const ECOLOR_FORMAT format)
 {