GPUMaterial: Fix Material appear broken when using sliders to tweak props

This was likely caused by some sort of race condition where the drawing
thread would request the state of the shader before the shader has been
compiled.

source/blender/gpu/intern/gpu_codegen.c

@@ -1894,9 +1894,8 @@ void GPU_nodes_prune(ListBase *nodes, GPUNodeLink *outlink)
 
 static bool gpu_pass_is_valid(GPUPass *pass)
 {
-	/* Shader is not null if compilation is successful,
-	 * refcount is positive if compilation as not yet been done. */
-	return (pass->shader != NULL || pass->refcount > 0);
+	/* Shader is not null if compilation is successful. */
+	return (pass->compiled == false || pass->shader != NULL);
 }
 
 GPUPass *GPU_generate_pass_new(