Warn when trying to enable TAA from mobile or gl_compatibility backend

TAA should only be enabled from the forward_plus renderer for now

servers/rendering/renderer_viewport.cpp

@@ -1138,6 +1138,7 @@ void RendererViewport::viewport_set_screen_space_aa(RID p_viewport, RS::Viewport
 void RendererViewport::viewport_set_use_taa(RID p_viewport, bool p_use_taa) {
 	Viewport *viewport = viewport_owner.get_or_null(p_viewport);
 	ERR_FAIL_COND(!viewport);
+	ERR_FAIL_COND_EDMSG(OS::get_singleton()->get_current_rendering_method() != "forward_plus", "TAA is only available when using the Forward+ renderer.");
 
 	if (viewport->use_taa == p_use_taa) {
 		return;