fix(ios): use decal shadows on Metal

lib/ShadowMap.lua

@@ -221,6 +221,9 @@ end
 -- where the canvas cannot be made -- VoxelScene then keeps the flat decal
 -- shadows, which need nothing but a quad.
 function ShadowMap.available()
+  if love.system and love.system.getOS and love.system.getOS() == "iOS" then
+    return false
+  end
   if not (love.graphics and love.graphics.newCanvas
           and love.graphics.setDepthMode) then
     return false