kernel4: replace continue in switch to support D3D11 shader model 5.0

Without this change, the fxc.exe compiler complains

error X3708: continue cannot be used in a switch

Signed-off-by: Elias Naur <mail@eliasnaur.com>

piet-gpu/shader/kernel4.comp

@@ -231,7 +231,7 @@ void main() {
         case Cmd_Jump:
             cmd_ref = CmdRef(Cmd_Jump_read(cmd_alloc, cmd_ref).new_ref);
             cmd_alloc.offset = cmd_ref.offset;
-            continue;
+            break;
         }
     }