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/Bush/assets/plant_bush.glb.import

@@ -0,0 +1,37 @@
+[remap]
+
+importer="scene"
+importer_version=1
+type="PackedScene"
+uid="uid://c3objh5he4fv7"
+path="res://.godot/imported/plant_bush.glb-4eaf384fbf61c7e3b82505202bc9874e.scn"
+
+[deps]
+
+source_file="res://Entities/Bush/assets/plant_bush.glb"
+dest_files=["res://.godot/imported/plant_bush.glb-4eaf384fbf61c7e3b82505202bc9874e.scn"]
+
+[params]
+
+nodes/root_type=""
+nodes/root_name=""
+nodes/apply_root_scale=true
+nodes/root_scale=1.0
+nodes/import_as_skeleton_bones=false
+nodes/use_node_type_suffixes=true
+meshes/ensure_tangents=true
+meshes/generate_lods=true
+meshes/create_shadow_meshes=true
+meshes/light_baking=1
+meshes/lightmap_texel_size=0.2
+meshes/force_disable_compression=false
+skins/use_named_skins=true
+animation/import=true
+animation/fps=30
+animation/trimming=false
+animation/remove_immutable_tracks=true
+animation/import_rest_as_RESET=false
+import_script/path=""
+_subresources={}
+gltf/naming_version=1
+gltf/embedded_image_handling=1