Metal: Disable MetalFX Temporal for mobile rendering method

It isn't supported, as `mobile` can't provide motion vectors. We can
revisit once it is supported via #100283.

Author: stuartcarnie

servers/rendering/renderer_viewport.cpp

@@ -971,8 +971,10 @@ void RendererViewport::viewport_set_use_xr(RID p_viewport, bool p_use_xr) {
 void RendererViewport::viewport_set_scaling_3d_mode(RID p_viewport, RS::ViewportScaling3DMode p_mode) {
 	Viewport *viewport = viewport_owner.get_or_null(p_viewport);
 	ERR_FAIL_NULL(viewport);
-	ERR_FAIL_COND_EDMSG(p_mode == RS::VIEWPORT_SCALING_3D_MODE_FSR && OS::get_singleton()->get_current_rendering_method() != "forward_plus", "FSR1 is only available when using the Forward+ renderer.");
-	ERR_FAIL_COND_EDMSG(p_mode == RS::VIEWPORT_SCALING_3D_MODE_FSR2 && OS::get_singleton()->get_current_rendering_method() != "forward_plus", "FSR2 is only available when using the Forward+ renderer.");
+	const String rendering_method = OS::get_singleton()->get_current_rendering_method();
+	ERR_FAIL_COND_EDMSG(p_mode == RS::VIEWPORT_SCALING_3D_MODE_FSR && rendering_method != "forward_plus", "FSR1 is only available when using the Forward+ renderer.");
+	ERR_FAIL_COND_EDMSG(p_mode == RS::VIEWPORT_SCALING_3D_MODE_FSR2 && rendering_method != "forward_plus", "FSR2 is only available when using the Forward+ renderer.");
+	ERR_FAIL_COND_EDMSG(p_mode == RS::VIEWPORT_SCALING_3D_MODE_METALFX_TEMPORAL && rendering_method != "forward_plus", "MetalFX Temporal is only available when using the Forward+ renderer.");
 
 	if (viewport->scaling_3d_mode == p_mode) {
 		return;