fix(render3d): lakes clipped to their ellipse; r3d_fog_base

A water body other than the reflecting one drew its whole bounding rectangle,
so the corners outside the lake's carved ellipse showed water over dry ground;
those bodies now discard outside the ellipse. r3d_fog_base (default 0) is the
height the height fog is measured from, so maps sharing one datum at different
heights get the same air.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/ludic.render3d/water.ludic

@@ -177,6 +177,10 @@ function water_draw(depth_tex: int) -> void {
     var r = F_ZERO
     if i == wb_primary { r = ron }
     u_f(gl_uniform(p, "u_refl_on"), r)
+    # every body but the mirrored sea is clipped to the ellipse inside its bounds
+    var clip = F_ONE
+    if i == wb_primary { clip = F_ZERO }
+    u_f(gl_uniform(p, "u_clip_ellipse"), clip)
     mesh_draw(water_mesh)
   }
   gl_disable(GL_BLEND)