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

src/client/renderingengine.cpp

@@ -179,7 +179,10 @@ RenderingEngine::RenderingEngine(MyEventReceiver *receiver)
 
 	// bpp, fsaa, vsync
 	bool vsync = g_settings->getBool("vsync");
-	bool enable_fsaa = g_settings->get("antialiasing") == "fsaa";
+	// Don't enable MSAA in OpenGL context creation if post-processing is enabled,
+	// the post-processing pipeline handles it.
+	bool enable_fsaa = g_settings->get("antialiasing") == "fsaa" &&
+			!g_settings->getBool("enable_post_processing");
 	u16 fsaa = enable_fsaa ? MYMAX(2, g_settings->getU16("fsaa")) : 0;
 
 	// Determine driver