Adds bushes and flowers to ground tiles

Implements bush and flower spawning on ground tiles based on vegetation density.
Adds new assets for bushes and flowers, and introduces multi-mesh rendering for optimized performance.
Introduces seasonal color variations for vegetation using a shader for bushes and materials for flowers and grass.
Refactors material application into a MaterialManager to handle material assignments over multiple frames.
Moves ground tile scripts into a subfolder.
Adds floating particles to test scene.

Utilities/MaterialManager/MaterialManager.gd

@@ -0,0 +1,30 @@
+class_name MaterialManagerClass
+extends Node
+
+var material_application_queue: Array = []
+var max_materials_per_frame: int = 20 # Adjust based on performance
+
+func _process(_delta):
+	# Process material applications gradually over multiple frames
+	process_material_queue()
+
+func process_material_queue():
+	var processed = 0
+	while material_application_queue.size() > 0 and processed < max_materials_per_frame:
+		var task = material_application_queue.pop_front()
+
+		if is_instance_valid(task.mesh_instance) and task.mesh_instance != null:
+			apply_material_immediately(task.mesh_instance, task.surface_index, task.material)
+		
+		processed += 1
+
+func queue_material_application(mesh_instance: MeshInstance3D, surface_index: int, material: StandardMaterial3D):
+	material_application_queue.append({
+		"mesh_instance": mesh_instance,
+		"surface_index": surface_index,
+		"material": material
+	})
+
+func apply_material_immediately(mesh_instance: MeshInstance3D, surface_index: int, material: StandardMaterial3D):
+	if is_instance_valid(mesh_instance):
+		mesh_instance.set_surface_override_material(surface_index, material)