feat: add grass and terrain

src/app/shaders/grass.vert

@@ -0,0 +1,25 @@
+float worldX = instanceMatrix[3][0];
+float worldZ = instanceMatrix[3][2];
+
+float windSpeed = 0.3;
+float windScale = 0.04;
+
+vec2 windPos = vec2(worldX, worldZ) * windScale;
+vec2 windFlow = windPos + vec2(uTime * windSpeed, uTime * windSpeed * 0.7);
+
+float windStrength = fbm(windFlow);
+windStrength = windStrength * 0.6 + 0.3;
+
+float turbulence = fbm(windPos * 3.0 + uTime * 2.0) * 0.15;
+
+vec2 windDir = vec2(
+    fbm(windPos * 0.5 + vec2(uTime * 0.1, 0.0)),
+    fbm(windPos * 0.5 + vec2(0.0, uTime * 0.1))
+);
+windDir = normalize(windDir - 0.5) * 0.8;
+
+float swayAmount = (windStrength + turbulence) * uv.y * uv.y;
+
+transformed.x += swayAmount * windDir.x;
+transformed.z += swayAmount * windDir.y;
+transformed.y -= abs(swayAmount) * 0.2;