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/EDriverFeatures.h

@@ -129,6 +129,9 @@ enum E_VIDEO_DRIVER_FEATURE
 	//! Support for clamping vertices beyond far-plane to depth instead of capping them.
 	EVDF_DEPTH_CLAMP,
 
+	//! Support for multisample textures.
+	EVDF_TEXTURE_MULTISAMPLE,
+
 	//! Only used for counting the elements of this enum
 	EVDF_COUNT
 };