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.h

@@ -227,6 +227,10 @@ public:
 	virtual ITexture *addRenderTargetTexture(const core::dimension2d<u32> &size,
 			const io::path &name, const ECOLOR_FORMAT format = ECF_UNKNOWN) override;
 
+	//! Creates a multisampled render target texture.
+	virtual ITexture *addRenderTargetTextureMs(const core::dimension2d<u32> &size, u8 msaa,
+			const io::path &name, const ECOLOR_FORMAT format = ECF_UNKNOWN) override;
+
 	//! Creates a render target texture for a cubemap
 	ITexture *addRenderTargetTextureCubemap(const irr::u32 sideLen,
 			const io::path &name, const ECOLOR_FORMAT format) override;
@@ -410,6 +414,8 @@ public:
 	//! Create render target.
 	IRenderTarget *addRenderTarget() override;
 
+	void blitRenderTarget(IRenderTarget *from, IRenderTarget *to) override {}
+
 	//! Remove render target.
 	void removeRenderTarget(IRenderTarget *renderTarget) override;