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/Tree/assets/temp/plant_bushDetailed.glb.import

@@ -1,37 +0,0 @@
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-[deps]
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-[params]
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