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/OpenGL3/Driver.cpp

@@ -69,6 +69,7 @@ void COpenGL3Driver::initFeatures()
 
 	AnisotropicFilterSupported = isVersionAtLeast(4, 6) || queryExtension("GL_ARB_texture_filter_anisotropic") || queryExtension("GL_EXT_texture_filter_anisotropic");
 	BlendMinMaxSupported = true;
+	TextureMultisampleSupported = true;
 
 	// COGLESCoreExtensionHandler::Feature
 	static_assert(MATERIAL_MAX_TEXTURES <= 16, "Only up to 16 textures are guaranteed");