update floor texture for default ground plane

Author: ruziniuuuuu

assets/synthnova_assets/default_ground_plane.xml

@@ -4,11 +4,7 @@
     <asset>
         <texture
             type="2d"
-            builtin="checker"
-            width="512"
-            height="512"
-            rgb1="0 0 0"
-            rgb2="1.0 1.0 1.0"
+            file="floor_texture.png"
             name="texplane"
         />
         <material