Implement asynchronous transfer queues, thread guards on RenderingDevice. Add ubershaders and rework pipeline caches for Forward+ and Mobile.

- Implements asynchronous transfer queues from PR #87590.
- Adds ubershaders that can run with specialization constants specified as push constants.
- Pipelines with specialization constants can compile in the background.
- Added monitoring for pipeline compilations.
- Materials and shaders can now be created asynchronously on background threads.
- Meshes that are loaded on background threads can also compile pipelines as part of the loading process.

scene/resources/particle_process_material.cpp

@@ -2261,6 +2261,8 @@ void ParticleProcessMaterial::_bind_methods() {
 
 ParticleProcessMaterial::ParticleProcessMaterial() :
 		element(this) {
+	_set_material(RS::get_singleton()->material_create());
+
 	set_direction(Vector3(1, 0, 0));
 	set_spread(45);
 	set_flatness(0);