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/bushes.gd

@@ -0,0 +1,36 @@
+# GrassController.gd
+extends Node3D
+class_name BushController
+
+@onready var bush_multimesh: MultiMeshInstance3D = $BushMultimesh
+var parent_node: GroundTile = null
+var bush_density: float = 0.5
+var bush_instance_range: int = 10
+
+func _ready() -> void:
+	parent_node = get_parent() as GroundTile
+
+func spawn_bushes_for_cell(value):
+	if value == null:
+		return
+	
+	bush_density = value.vegetation_density
+	update_bush_density()
+	
+	if bush_multimesh and bush_multimesh.has_method("setup_multimesh"):
+		bush_multimesh.setup_multimesh()
+
+func update_bush_density() -> void:
+	if parent_node == null:
+		return
+
+	var rng = parent_node.get_rng()
+
+	if bush_density > 0.8:
+			bush_instance_range = rng.randi_range(5, 10)
+	elif bush_density > 0.6:
+			bush_instance_range = rng.randi_range(2, 7)
+	elif bush_density > 0.3:
+			bush_instance_range = rng.randi_range(0, 1)
+	else:
+			bush_instance_range = rng.randi_range(0, 0)