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.

Entities/GroundTile/scripts/ground_tile.gd

@@ -0,0 +1,59 @@
+class_name GroundTile
+extends Node3D
+@onready var debug_text: Label = $DebugText/DebugTextViewport/DebugTextLabel
+@onready var tree_spawner = $Trees
+@onready var grass_spawner = $Grass
+@onready var bush_spawner = $Bushes
+@onready var flower_spawner = $Flowers
+@onready var ground = $Ground
+var grid_x: int
+var grid_z: int
+var cell_info: CellDataResource = null
+var spawners_ready: bool = false
+var cached_rng: RandomClass = null
+
+# 326432  229379
+# Grass - 1a7761 1f6051
+
+func _ready() -> void:
+	spawners_ready = true
+
+	if cell_info != null:
+		spawn_content()
+		update_text_label()
+
+	ground.material_override.albedo_color = ColorData.grass_materials[Season.current]['base']
+
+func get_rng() -> RandomClass:
+	if cached_rng == null and cell_info:
+		cached_rng = RandomClass.get_seeded_instance(cell_info.cell_seed)
+	elif cached_rng == null:
+		cached_rng = RandomClass.get_shared_instance()
+	
+	return cached_rng
+
+func set_grid_location(x, z) -> void:
+	grid_x = x
+	grid_z = z
+	cell_info = MapData.get_map_data(grid_x, grid_z)
+	
+	# Only spawn if spawners are ready
+	if spawners_ready and cell_info != null:
+		spawn_content()
+
+func spawn_content():
+	if cell_info == null:
+		return
+		
+	if tree_spawner:
+		tree_spawner.spawn_trees_for_cell(cell_info)
+	if grass_spawner:
+		grass_spawner.spawn_grass_for_cell(cell_info)
+	if bush_spawner:
+		bush_spawner.spawn_bushes_for_cell(cell_info)
+	if flower_spawner:
+		flower_spawner.spawn_flowers_for_cell(cell_info)
+	
+
+func update_text_label() -> void:
+	debug_text.text = str(cell_info.vegetation_density)