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/ground_tile.gd

@@ -1,49 +0,0 @@
-class_name GroundTile
-extends Node3D
-@onready var debug_text: Label = $DebugText/DebugTextViewport/DebugTextLabel
-@onready var tree_spawner = $Trees
-@onready var grass_spawner = $Grass
-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()
-
-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)
-
-func update_text_label() -> void:
-	debug_text.text = str(cell_info.vegetation_density)