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.

leaves.gdshader

@@ -0,0 +1,23 @@
+shader_type spatial;
+uniform vec4 base_color : source_color = vec4(0.2, 0.6, 0.1, 1.0);
+uniform vec4 variation_color : source_color = vec4(0.4, 0.8, 0.2, 1.0);
+uniform float gradient_height : hint_range(0.1, 5.0) = 2.0;
+uniform float gradient_offset : hint_range(-2.0, 2.0) = 0.0;
+
+varying vec3 world_position;
+
+void vertex() {
+    world_position = (MODEL_MATRIX * vec4(VERTEX, 1.0)).xyz;
+}
+
+void fragment() {
+    // Use world Y position instead of UV
+    float gradient_factor = (world_position.y + gradient_offset) / gradient_height;
+    gradient_factor = clamp(gradient_factor, 0.0, 1.0);
+    
+    vec3 final_color = mix(base_color.rgb, variation_color.rgb, gradient_factor);
+    
+    ALBEDO = final_color;
+    ROUGHNESS = 0.8;
+    METALLIC = 0.0;
+}