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/OpenGL/ExtensionHandler.h

@@ -74,6 +74,8 @@ public:
 			return false;
 		case EVDF_STENCIL_BUFFER:
 			return StencilBuffer;
+		case EVDF_TEXTURE_MULTISAMPLE:
+			return TextureMultisampleSupported;
 		default:
 			return false;
 		};
@@ -161,6 +163,7 @@ public:
 
 	bool AnisotropicFilterSupported = false;
 	bool BlendMinMaxSupported = false;
+	bool TextureMultisampleSupported = false;
 };
 
 }