feat: selection tracking

test-terrains/my_terrain.terrain.tres

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+[gd_resource type="SaveData" load_steps=10 format=3]
+
+[sub_resource type="PointPrimitive" id="PointPrimitive_pxqd5"]
+center = Vector2(5.4302, 139.65)
+slope = -0.7
+height = 67.0
+
+[sub_resource type="PointPrimitive" id="PointPrimitive_ba0ut"]
+center = Vector2(-81.1489, 92.763)
+slope = -0.7
+height = 35.0
+
+[sub_resource type="FastNoiseLite" id="FastNoiseLite_pxqd5"]
+frequency = 0.02
+fractal_octaves = 3
+
+[sub_resource type="NoisePrimitive" id="NoisePrimitive_ba0ut"]
+blend_range = 5.0
+noise = SubResource("FastNoiseLite_pxqd5")
+noise_amplitude = 20.0
+
+[sub_resource type="FastNoiseLite" id="FastNoiseLite_b1cmn"]
+noise_type = 0
+frequency = 0.03
+fractal_type = 2
+metadata/_preview_in_3d_space_ = true
+
+[sub_resource type="NoisePrimitive" id="NoisePrimitive_pxqd5"]
+blend_range = 10.0
+noise = SubResource("FastNoiseLite_b1cmn")
+noise_amplitude = 5.0
+
+[sub_resource type="FastNoiseLite" id="FastNoiseLite_ba0ut"]
+fractal_type = 2
+fractal_gain = 1.0
+fractal_weighted_strength = 0.58
+
+[sub_resource type="NoisePrimitive" id="NoisePrimitive_q68jb"]
+blend_mode = 1
+noise = SubResource("FastNoiseLite_ba0ut")
+
+[sub_resource type="PlanePrimitive" id="PlanePrimitive_pxqd5"]
+blend_range = 10.0
+baseline = -1.0
+
+[resource]
+primitives = [SubResource("PointPrimitive_pxqd5"), SubResource("PointPrimitive_ba0ut"), SubResource("NoisePrimitive_ba0ut"), SubResource("NoisePrimitive_pxqd5"), SubResource("NoisePrimitive_q68jb"), SubResource("PlanePrimitive_pxqd5")]